D.B.
My son graduated last year and the average gift was $20 or so in the form of a gift card. He really appreciated being able to buy things for college. Since you don't know the graduate, I wouldn't spend more than $25 tops. Good cards are discount department stores like WalMart or Target or Penney's, whatever is near the graduate, where he can buy room accessories (if he's going to college), clothes, CDs, sporting goods. Most of the kids are into music, so iTunes gift cards are appreciated too. They also like to go out with their friends before everyone scatters for college, so restaurant or movie cards are often a good gift.
If he is going to college, another nice thing is to get a gift card from the college bookstore where he can buy college shirts or accessories, or things for his room. You can do this online or by phone - the service is pretty good because there are few if any students on campus, and the schools are used to this type of request.
If you buy gift cards in a store, remember to buy the ones behind the cash register or courtesy counter. The pre-printed ones on the display racks elsewhere in the store are often useless - the crooks copy down the serial numbers and then keep trying to use them online. They take their chances but hope that, in periods of heavy buying (around graduation and holidays, for example), cashiers will activate cards which won't be used for a while because they are being given as gifts. They run into some dead ends but they get plenty of good hits too. Then the recipient goes shopping a month later and the card is "empty".
Good luck!
