Investing in coins and collectibles: Buffalo nickels and Spiderman comic books are good examples of coins and collectibles that can rise in value, and thus offer opportunity for passive income investors. You'll need to get up to speed on the value of any coin or collectible under consideration, but once you do so, you're on the way to price appreciation on a commodity you'll be paying a lower price to buy, and will garner a higher price when you sell.
Great breakout of some common items that are (mostly) accessible to individuals. My biggest issue with p2p is the ordinary interest it generates and the ordinary tax that we have to pay. That really takes a bite out of the returns. Fortunately, I opened an IRA with one of the providers to juice the return with zero additional risk. 6-8% nominal returns over a long period of time will make me very happy. It should end up as 5-7% of the portfolio anyway, so nothing too significant.
I have only dabbled in drop-shipping before when I had an eCommerce platform 6 years ago or so. I think it is something that you could do on the side, but you would want to do in depth research on the industry you want to get into before setting up shop. It may be a little less passive up front, but over time you could take your hands off the wheel.