If it's for a dude and he has some sort of liking for games, go for the xbox. Actually pay the difference to get a 120 GB PS3 Can't go wrong there. Slap a bundle with it so he is covered with something to play.
A lady of the night?
Guy of the night?
Start a trust fund
Hundreds of lotto tickets
Nu phone/part of one
Deposit on 2nd hand car.
Part of a scooter
And what the poster above said.
It really depends on the person who is receiving the gift. I just celebrated my 21st on Saturday, got some rad things, though I wouldn't be able to recommend them for everyone.