Quiddler and Pocket Edition Scrabble.

Quiddler is a card game which won a 1999 Parents' Choice Silver Award. From their review: "Make either the most words or the longest word from the letter cards dealt and by rummy-like drawing and discarding. This word-forming game uses a 118-card deck containing individual letters from A to Z, plus double-letter cards (QU, TH, IN, ER, CL). A game consists of eight rounds: players get three cards in the first round, four in the second, and so on up to ten cards in the final round. Players draw and discard as in rummy games until one is able to use all his or her cards to form one or more words. There is a bonus for the longest word in each round as well as for the most words, so children have as good a chance as adults to win. For one to eight players aged 8 and up."

Pocket Edition
Scrabble is great for trips. The pieces fit securely on the board which can even be folded up in the middle of a game.