The two that I enjoyed most were Starship Troopers and The Moon is a Harsh Mistress. I would recommend either of those, although Starship Troopers is a rather short novel... for a trip, you might want to take along The Moon is a Harsh Mistress instead.
listenhillary in post #85 listed a good bibliography of Heinlein's novels... I think you'd enjoy pretty much any of his novels up through Moon.... Also, I find that his so-called "juvenile" novels are good reads too, although short... that may be a result of the reading ability juveniles were expected to have in the 40s and 50s!