I have 2 mice in a 2ft tank...mine do not have a wheel as i really dont like the wire ones due to causing tail injuries, and i'm having trouble finding a different type at the moment. All my mice get natural tree branches to explore though and boxes (i change them around so they dont get bored.

All my mice a fed a basic mouse mix that my pet supply store makes mixed in with a guinea pig mix, as well as supplimentary feed as well, including fresh fruit and vegetables

Yep they can have the critter balls...you can get real small ones suitable for mice