Donating plasma can actually work out better than selling your blood. It takes longer to donate plasma because they will put your red blood cells back in your system. The upside is that you can donate plasma as much as twice a week and you can make anywhere from $40-$60 per donation. You can make that $100 in less than a week if you can stomach the needle.
Buy underpriced used books. Outfit a phone with an ISBN reading app, scan the ISBN numbers of books at used bookstores and thrift shops, and compare the asking prices with what the books are selling for on a site like Amazon. Whenever you get a good hit (which won’t be often but, since the process is fast, won’t take long, either), buy the book and resell it online. Be discreet about this, as the store managers probably won’t like what you’re doing.
If you're running on fumes, financially speaking, but you have some money coming your way soon, consider pawning something of value to borrow fast cash. Of course, to get those items back you'll need to pay back the loan with interest. If you don't pay it back in time, that you'll lose the item. If it's really something that has a lot of intrinsic value to you, don't do it. But if it's something that doesn't, you can certainly consider it depending on your situation.
I just pulled in $500 the other day by signing up for an American Express card for my wife, and a Chase Ink card for my business. I’m planning a trip and we are paying for it all by using the sign-up offers. There are also no annual fees on these cards. So I recommend taking advantage of a few signup offers and you’ll easily make $100 (if you qualify).
Here's a far easier way to receive passive income: Use cash-back or rewards credit cards. (If you're carrying a lot of debt or don't have the discipline to use credit cards responsibly, you're best off not using this income-generation method.) You can collect flat-rate cash-back percentages up to about 2% with lots of cards, while some cards offer more focused rewards or cash back based on certain kinds of spending or certain retailers.
If you’ve ever wanted to try real estate investing but don’t want to deal with all the stress of being a landlord, you might want to consider investing with Fundrise. Fundrise is a new platform that allows you to invest directly in a real estate portfolio that a team of professionals identifies, acquires, and manages on your behalf. With a starting investment as small as $500, you get exposure to dozens of solid, value-producing assets.