5 Hours huh? Let's see...Galcon is an interesting one, it's a simple Real Time Strategy game, each game lasts 2-5 minutes but it's VERY addictive. You should try out the free demo to see if you'd like it. If you can manage to have a good internet connection during the trip you can even play multiplayer online. Another good one for long periods of time is Chimps Ahoy, it's an interesting mix between Arkanoid and Pong (you control paddles on both sides of the screen to attack blocks and pirate ships and stuff), it has 100 levels and has that "Just one more stage!" addictiveness. There's a free demo for that too, only has a few stages. MotionX Poker is another addictive one - it's just playing poker with dice, casino style, but they bounce around realistically when you shake the device, and you've got goals to unlock other boards and dice and stuff.
These are the three games I play the most, that seem to have the most replayability even after lots of plays.