You can get/create choose-your-own-adventure type games using iPod Notes at iPod Arcade, you can get/create new iQuiz quizzes using the resources at iQuizShare.
Apple don't release an SDK, and encrypt and sign binaries for the native games sold on the iTunes store. Ditto for a 6G's firmware, so you can't simply install Linux as with earlier models. Lame, huh?