13 Answers

Any kind of electronic gizmo, perhaps a cell phone, I pod...

A few cases of SPAM and explain this is the only food they'll be able to afford if the world doesn't get a handle on the money problem.

$100 bill is good.

I know what you mean! it hard to find something appropriate at that age, these day kid have just about everything you can think of, money is about the only thing you can give them, that way they can choose for them self.

I lol at Spam but have granddaughter turning 18 this year giving her a dinner party and jewelry case.

Give her or him what she likes. A bracelet or some grown-up stuff could be good.

I really don't like to give money as a gift.  I always feel like I can come up with a gift that is "personal" and special.  For a girl, a pedicure and/or manicure, or spa day would be fun. If you can afford it for her and a friend, so much the better.
For a guy, I would consider a gift certificate for a nice restaurant. If he played golf, perhaps a round of golf at a nice course. Golf balls, tees, and other assorted miscellaneous is nice.  If tennis is the game, tennis balls, tennis hat, wrist things.  
A car safety kit is a good gift.....some first aid stuff, flares, flashlight, jumper cables.
A membership to AAA is nice for anybody. (One Christmas, my kids got jumper cables and the AAA membership.)  
If there is a hobby, you can feed the hobby needs.  

Male of female?

Are you 18 yrs old already or you just wanna give your 18 sister a present for her birthday but I prefur you to give her what chiangmai said.

How about a gift card for their fave fashion store..or ipod token...you can

It depends on the relationship of you and this 18 year old.

A friend, you know what they like, dislike. Video games, whatever.

A boyfriend something more personal. bracelet, necklace, cell phone.

A sorta friend, gift card.

A goodly supply of condoms.

money