I used to feed all sorts of things to the rats. I also gave them dog chew bones(edible ones, not plastic ones), and Petsmart also has wooden blocks for rabbits and rats to chew on that the like too.

For fun, you can give them a whole apple and watch them "hide it" in the corner by tossing bedding on top, then get confused when one of the other rats "finds" it despite all that effort to hide it.