If you like text adventures, let me recommend Frotz, an open-source Z-machine interpreter for iDevices. You can download hundreds of text adventure games, and it is just like playing infocom games of the past.
Except for downloading new games, you can use Frotz in Airplane mode. I particularly find text adventures useful for helping me build up typing speed and skill on the integrated keyboard. I often play a game in the para transit van during my long commute and I haven't gotten around to needing an external keyboard for my iPod yet, simply because I got so much typing practice that way.
Frotz gives you access to the interactive fiction archive, so you don't need to transfer games to it via iTunes. You simply browse through the archive and download any game that sounds interesting. Once the game is on your device you can play it any time without further need to access a network connection.