One thing about Japan is that it is a land of vending machines. And the cans that often come out of those machines have stickers on them for prize contests. Basically, you collect so many stickers from the cans, send them in and if your card is drawn, you win a prize.
While I get a few hundred dollars worth of stuff from these contests every year, I don't spend a cent (or yen in my case) collecting these stickers.
I have gotten into the habit of picking up cans thrown on the street and throwing them away properly (an old habit taught to me by my father to always leave any place you go better than when you arrived). If those cans have a sticker on it (which they often do), I take it as my payment for cleaning up.
The prizes in Japan can be quite good -- my winter coat, spring jacket and portable CD player are all prizes just to name a few. So in the end I get some good exercise (walking), make the area a bit cleaner, and get cool things for my effort. Seems like a good deal to me.