You can try some mini-sized versions of ordinary games. I have a travel version of Scrabble - the letter tiles snap into place to keep them from shifting. I also have a travel Poker set with chips in tubes.
Maybe you can buy them a Nintendo, mostly that hepls. It isn't educational right but in this age they don't want something else. Maybe an I-Pod will help, too.
Any hand held electronic game is usually the best bet. If you have something like a nintendo DS this will entertain teenagers for hours. If you dont then there are cheap hand held games or even card games can be entertaining for shorter trips of maybe 2 hours or less.
