There is a small animals chew block that looks like a big chunk of chaulk. You can attach the whole thing to the cage, or break them up with a hammer and give them hand size peices to snatch and run with. They have calcium and minerals in them too.
Also try tums, a good chew with calcium.
Grape vines from the craft shop are fun, they come in so many sizes. I have a big grape vine wreath hung up for when they are out for their run. I buy small ones for in the cage.
Apple twigs, washed and dried and popsycle sticks are a favorite too.
As Jo ann has said always keep safty in mind, watch out for sharpies, plastics, soft metals, and finishes.