Hashime said:
SCUBA..... Snorkeling!
Just get some used fins, a mask, and a snorkel.
Don't go looking for the super good stuff, I spent $90 on my SCUBA mask alone, plus it was $50 for my snorkel. You can buy cheap ones for about $35 combined. Fins are a LOT of expensive (Decent ones are $175) but you don't need super good ones until you decide you want to keep it up as a hobby. Besides, the bad ones make you work really hard, and that helps you build up muscles in your legs and burn calories.
Under NO circumstances take up car rebuilding as a hobby. I got my project car for $250, but I am $2100 into repairs, and she is STILL not roadworthy. It is fun though...