Here is a list of 90 things about passive income, each with a brief explanation. The order of the list has been randomized for variety.

1. Rental properties: Investing in real estate and earning rental income is a popular form of passive income.
2. Dividend stocks: Certain stocks pay dividends, which are regular payments made to shareholders as a portion of the company's profits.
3. Peer-to-peer lending: Online platforms allow you to lend money to individuals or businesses and earn interest.
4. Digital products: Creating and selling digital products like e-books, courses, or software can generate passive income.
5. Affiliate marketing: Promoting other people's products or services and earning a commission for each sale or referral.
6. Royalties: If you have intellectual property rights, such as patents, copyrights, or trademarks, you can earn royalties from their use.
7. Create an app: Developing a smartphone app that generates revenue through ads or in-app purchases.
8. Print-on-demand: Designing and selling custom merchandise online without the need for inventory.
9. Create an online course: Sharing your expertise through an online course and earning revenue from course sales.
10. YouTube channel: Creating and monetizing a YouTube channel by producing videos and earning from ads or sponsorships.
11. E-commerce: Running an online store that sells physical products, either through dropshipping or inventory management.
12. Renting out your space: Listing your spare room, parking spot, or storage space on platforms like Airbnb or Neighbor for extra income.
13. Create a blog: Writing and monetizing a blog through advertising, sponsored content, or affiliate marketing.
14. High-yield savings account: Earning interest on your savings deposited in a high-yield savings account.
15. Index funds: Investing in low-cost index funds that track the performance of a specific market index.
16. Cryptocurrency staking: Holding and validating certain cryptocurrencies to earn additional coins as rewards.
17. Automated online store: Setting up an e-commerce store that is fully automated using dropshipping and marketing automation tools.
18. Vending machines: Owning and managing vending machines that generate revenue through product sales.
19. Licensing music: Composing and licensing music for use in commercials, films, or other media.
20. Self-publishing: Writing and self-publishing books or e-books through platforms like Amazon Kindle Direct Publishing.
21. Affiliate niche websites: Building niche websites that generate income through affiliate marketing.
22. Create an online marketplace: Establishing an online platform that connects buyers and sellers and earns a commission on each transaction.
23. Stock photography: Selling your photographs online through stock photography platforms and earning royalties.
24. Network marketing: Joining a reputable network marketing company and earning passive income from sales made by your downline.
25. Create a membership site: Offering exclusive content or services through a membership website and charging a recurring fee.
26. ATM machines: Owning and operating ATM machines to earn transaction fees.
27. Rental equipment: Purchasing equipment and renting it out to others for a fee.
28. Real estate crowdfunding: Investing in real estate projects through online crowdfunding platforms.
29. Create a software-as-a-service (SaaS): Developing and selling a software product that customers can access through a subscription model.
30. Affiliate partnerships: Building partnerships with complementary businesses to earn referral fees or commissions.
31. Peer-to-peer car sharing: Renting out your car through platforms like Turo or Getaround when you're not using it.
32. Storage unit rentals: Owning storage units and renting them out to individuals or businesses.
33. Online market trading: Engaging in stock, forex, or cryptocurrency trading to generate passive income from capital gains.
34. Buy an existing business: Purchasing an established business that generates passive income under capable management.
35. Create an online directory: Developing an online directory of businesses or professionals and charging a fee for listings.
36. Buy an existing website: Acquiring an established website with traffic and monetization potential.
37. Create a software plugin: Designing and selling plugins or extensions for popular software platforms.
38. Stock options: Investing in stock options that provide the right to buy or sell shares at a predetermined price.
39. Create an online tool: Building a useful online tool or calculator and monetizing it through ads or subscriptions.
40. Purchase a franchise: Buying a franchise business that generates income from ongoing royalty payments.
41. Self-storage units: Constructing and renting out self-storage units for individuals or businesses.
42. Automated investing: Using robo-advisors or automated investment platforms to manage your investments.
43. Create a podcast: Producing a podcast and monetizing it through sponsorships or advertising.
44. Advertise on your car: Placing advertisements on your car and getting paid for driving as usual.
45. Create a digital marketplace: Developing an online platform where users can buy and sell digital products.
46. Dropshipping from China: Running an e-commerce business by sourcing products directly from Chinese suppliers.
47. Online surveys: Participating in paid online surveys or market research studies.
48. Rent out your belongings: Leasing out items you own but rarely use, such as cameras, tools, or recreational equipment.
49. Real estate investment trusts (REITs): Investing in publicly traded companies that own and manage income-generating properties.
50. Buy dividend-paying ETFs: Investing in exchange-traded funds (ETFs) that focus on dividend-paying stocks.
51. Create a mobile game: Developing a mobile game and monetizing it through ads, in-app purchases, or subscriptions.
52. Create an online platform: Building a website or app that connects service providers with customers and earns a commission on transactions.
53. Rent out billboard space: Owning or leasing billboard space and generating income from advertising.
54. Peer-to-peer energy trading: Generating renewable energy at home and selling the excess to neighbors or the grid.
55. Create an online coaching program: Offering coaching or mentoring services online and charging clients for access.
56. Buy tax liens: Investing in tax liens and earning interest or acquiring property if the liens are not redeemed.
57. Create an online marketing agency: Providing marketing services to businesses online and earning a percentage of the advertising budget.
58. Passive income from patents: Licensing your patented inventions to companies in exchange for royalties.
59. Build an email list: Growing an email list and monetizing it through affiliate offers, sponsored content, or product sales.
60. Content syndication: Allowing other websites or platforms to republish your content for a fee.
61. Create an online travel agency: Offering travel services online and earning a commission on bookings.
62. Pay-per-click advertising: Monetizing your website or blog by displaying targeted ads and earning revenue for each click.
63. Create an online language course: Developing and selling an online language learning course.
64. Create a comparison website: Building a website that compares prices or features of products or services and earns referral fees.
65. Sell stock photos on your own website: Setting up your own website to sell your photography and keeping all the profits.
66. Create a YouTube course: Teaching others how to grow a successful YouTube channel and selling the course online.
67. Passive income from trademarks: Registering trademarks and licensing their use to other businesses for a fee.
68. Create an audiobook: Recording and selling audiobook versions of popular books.
69. Build an online community: Creating an online forum or social platform and monetizing it through memberships or ads.
70. Create an online marketplace for freelancers: Building a platform where freelancers can offer their services and earn a commission on transactions.
71. Create a premium content platform: Offering exclusive content or resources behind a paywall and charging a subscription fee.
72. Rent out equipment for events: Providing equipment like sound systems, projectors, or party supplies for events.
73. Peer-to-peer lending for small businesses: Lending money to small businesses through online platforms and earning interest.
74. Create an online fitness program: Developing and selling a fitness program or workout videos online.
75. Licensing software: Creating software applications and licensing them to businesses or individuals.
76. Create a digital marketing course: Teaching others how to effectively market their businesses online and selling the course.
77. Create an online stock trading course: Educating others on stock trading strategies and selling the course online.
78. Create a passive income blog: Sharing tips and strategies for generating passive income and monetizing the blog.
79. Rent out office space: Leasing out office space or co-working spaces to businesses or entrepreneurs.
80. Create an online resume writing service: Offering professional resume writing services and charging a fee for each resume.
81. Create a passive income podcast: Producing a podcast that explores different passive income strategies and monetizing it through sponsorships.
82. Rent out filming locations: Offering unique or picturesque properties as filming locations for movies or commercials.
83. Create a dropshipping course: Teaching others how to start a successful dropshipping business and selling the course online.
84. Licensing artwork: Creating artwork and licensing its use in various products or marketing materials.
85. Invest in farmland: Purchasing agricultural land and leasing it to farmers for crop production.
86. Create a membership-based content platform: Offering exclusive content, articles, or videos behind a membership paywall.
87. Rent out outdoor advertising space: Owning or leasing billboards, bus stops, or other outdoor advertising space.
88. Create an online graphic design service: Providing graphic design services to clients online and earning a fee for each project.
89. Create a virtual assistant agency: Building a team of virtual assistants and connecting them with businesses that need their services.
90. Passive income from affiliate networks: Joining affiliate networks that offer a wide range of products and earning commissions on sales generated through your affiliate links.

Remember, passive income requires initial effort and investment but can provide ongoing returns with less active involvement over time. Each of these methods offers potential opportunities to generate income passively, but it's important to research, plan, and execute them appropriately to achieve success.