My kids have a little travel mancala and cribbage board. That, a deck of cards and a Go board made out of a bandana has kept us amused for years. Well, did, I need to make a new bandana, my Go grid looks a little threadbare these days.
They used to bring Uno along too but we really didn't play it much. I brought Boggle on our last couple of trips, because we're using it as a vocabulary exercise. We don't time it or write it down, we just yell out words as we find them and don't really keep score.