Stimulating Toys for Rabbits

My rabbit loves ANYTHING cardboard and places where she can hide her head, pretend she's asleep and then bink out backwards just as I'm about to take a sip of hot tea (everytime it seems!).

I wonder if there are other inexpensive and homemade things that i can give her to stimulate her intelligence and sense of fun.

I always gave my rabbits smooth plastic balls to play with. The balls were of good quality and colorful.

The does would roll them around in their cages and the buck, well he had another use for his ball. Of course being a buck he was a bit in the mood for love all the time, so his pretty ball got used a lot.

I also hung bells in the does cages and they seemed to enjoy the soft tingling noises. I never gave them anything small enough that they could swallow, chew or anything with sharp edges.