Counterfeit money is something everyone encounters every now and then. It doesn't matter how you came across it. It is in your possession, and you need to do something with it. The "right" thing to do is turn it over to the police, or perhaps just to destroy it. However, it seems there are other promising options available for your fake money.
Take it down to the church of your choice and put it in the collection plate. It doesn't need to be your own church. You might be a Catholic and want to teach the Lutherans down the street a lesson. Or maybe you just want to screw over the Scientology center for harassing you a bit too much. No matter what church you take it too, you can still write it off on your taxes. Don't feel bad about it, they sell false hope at churches, so why not pay for it in false money?
Give it too people that are looking to make change. "Hey man, got change for a twenty?", "Why yes I do". Admittedly this only works if you have small bills that have been counterfeited, and a lot of them. Of course you could do the reverse of this, stand out side a 7/11 looking to break a 50 or 100 because most of them won't take large bills.
Take it down to a "swap meet". A "swap meet" is a wonderful place where people sell stolen and knock-off merchandise off of blankets and tables. As almost everything is already stolen, it seem like the perfect place to spend a few counterfeit dollars. This way if what you bought doesn't work, you aren't really out anything.
Just a few ideas. You should of course do the "right" thing. Even if it is not the funny option.