I've had this happen on a few occasions - we even told people for our wedding not to buy gifts or give money or anything!!!
Anyway, we had to go to a friends wedding the other weekend, and they asked for cash - which is so much the norm now it didn't worry us in the slightest. So we just gave them $100 in a card - and people "usually" have a wishing well there that you just pop it into. You don't even have to acknowledge the amount that you give them, just chuck the money in an envelope and then have your card separate if you don't want them to know how much money you are giving them
Good luck - and congrats to the happy couple..