Another benefit of investing in rental properties is the loan pay down. If you obtain a loan to buy the property, each month your tenants are paying off part of the loan. Once the mortgage on the property has been paid off, your cash flow will increase dramatically, allowing your mediocre investment to skyrocket into a full-fledged retirement program.
Why did P2P lending get a liquidity ranking of 6? It is quite possibly the most illiquid investment option you listed. You said you rank liquidity by “difficulty level of withdrawing your money without a massive penalty”, and for Lending Club notes, it’s not only difficult and extremely time consuming to sell all of your notes in their super illiquid market, but you would have to sell your notes at large losses to hope to get others interested in buying your notes. On top of that, it is impossible to withdraw your money any other way other than just waiting for interest/principal to pay off every month until maturity in 3 to 5 years. You can’t just one day tell Lending Club “I want to quit, please give me my money back.” One can even argue that it is less difficult to sell a home (in order to “withdraw” the money invested) than to withdraw all of their money from a P2P loan portfolio because it is very possible to sell a home before 3 to 5 years.
You can find dividend stocks using Google Finance Stock Screener which is free to use. Set the search criteria for the P/E Ratio, and Dividend yield (shown as a percentage) criteria. You can set minimum and maximum values; in the dividend yield box, set it between 2 and 100. This will search for stocks that pay dividends worth between 2-100% of the current stock price.
Investing in Index Funds – I use Vanguard for this, but there are several reputable sites out there that allow you to do the same thing.  It’s a good way to invest excess cash that you don’t need now and use it to diversify your portfolio.  I’m not going to make a specific recommendation here, but Vanguard does have a page that will make a recommendation to you based on your risk tolerance.  This is generally going to require more up-front money than Lending Club (probably $1,000+), but if you have the money, it’s something to consider.
If you have an empty house or room you can rent it out on AIRBNB and OYO Rooms. Many travelers are looking to spend one night at a place. You can always rent out your empty house or room to them. All you need to do is list your room or house online, explain the rules and you are good to go. Travelers will pay you online. This way you don’t have to search for clients. They will come to you.

Sell Digital Files on Etsy – I’ve been into home décor lately and I had to turn to Etsy to find exactly what I wanted. I ended up purchasing digital files of the artwork I wanted printed out! The seller had made a bunch of wall art, digitized, and listed it on Etsy for instant download. There are other popular digital files on Etsy as well such as monthly planners. If you’re into graphic design this could be an amazing passive income idea for you.