They would probably prefer a fiver in a card, that way if other people give them money they can put it together and buy a game or make up or something.
That really isn't very nice.
A gift is a token of love the amount is quite immaterial, and you don't know someone elses circumstances.
I would be very pleased if someone were to spend a fiver on me, and I'm not kidding.
A cinema voucher - I always buy teenagers them at Christmas/birthdays - the seem to like it and you know its not going to be a waste and thrown away etc.