I can now report that Dragon Quest IX has the richest post-game of any Dragon Quest game. Which is great because it's on a hand-held, and it's likely to provide me hours of hours of entertainment on several trips I'll be taking over the next few months. Basically once you beat it, it turns into a Nethack type game with random dungeons, bosses, and loot. You can unlock bosses from previous Dragon Quest games, and there's a virtually unlimited amount of random dungeons that can be generated. It's pretty neat.