Mechanical Turk: Amazon's Mechanical Turk is a resource for doing human-intelligence tasks, or as the site commonly refers to them, HITs. You get paid a very small fee for any given HIT and you'll need a good deal of volume to make a substantial amount of money. But it is a resource you can use in your spare time to generate a small income online. 
Amazon makes it fairly easy to list and sell old books on its marketplace. You can make more than just a few bucks If you have pricey text books from college. Be sure the books are in good condition. You'll get negative reviews if you attempt to sell books that are falling apart. Remember, add value where you can, no matter how small it might be and no matter how few people might readily notice it. 
How much will you make? I think my best month with Google AdSense was almost $5,000 over the last ten years. That amazing month blew my mind since it was actually near the beginning of my blogging journey. When you go from making zero to $5,000 in a month, that will rock your world. For me, it also got me even more excited because I knew there were other ways to monetize.