Things to bring on a carry on bag?

Near the end of November, I will be going down to Disney World with the band and choir. Now, I live in Pennsylvania and it'll be at least a two day trip. (Who knows if something will happen on the way down like a flat tire?) Well, the teachers said we should bring a carry-on bag. I have no clue what to take on though! We will be riding on a coach bus and only stopping to get food for breakfast, lunch, and dinner. It's not the first time I had ridden on a coach bus but it was NEVER over night. The things I know I WILL be taking are: cell phone, ipod, books, and snacks. I don't know what else to bring though.. any suggestions? Note: The bathrooms on the buses don't have sinks! (Only hand sanitizer) Another thing, what should I use for my carry on bag? Should I just use a backpack?