A while back I mentioned a few things you can do with counterfeit money. It is not a common thing to run across for most people, most people would not realize they had it and just keep passing it on. However if you do know you have some, why not have a little fun?
You can use it to screw with people. The various groups that camp out in front of stores are a good target. The girl scouts are a good group to pass the money on too, and you get some cookies. The salvation army man ringing his bell, well reward his ever annoying noise with some of your newly found counterfeit currency. Someone taking donations for a political cause you don't like, can you think of a better target? Of course, all of these groups are doing things legally, perhaps it would be better to pass it to someone working illegally.
It is after all illegal to spend counterfeit money; so why not spend it on something illegal to begin with? Got a drug problem, make it less of a problem with funny money. Need some stress relief? Get your fake cash together and find a hooker with big fake tits. If you want to put a hit on someone, think about how you could pay for it. Admittedly, most illegal items would take an unusually large amount of counterfeit money.
Now if you are not in the mood for fun, you could always turn your counterfeit money into the proper authorities. Not sure why someone would do that, but you could. You didn't make the money, but you can still have fun with it.