Donating plasma (for those not familiar, part of your blood that is easily replaced), while not pleasant, is easy & can earn you a good amount of money per session.
Aventis is as good as any. You may have to wait, and it's not pleasant sitting with your arm hooked up to what amounts to a blood-plasma-extracting machine, but it's easy enough and if you don't mind the thought, can earn you some extra money.
I myself only donated once or twice due to being a little uneasy having myself hooked up, but YMMV.
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