i provide cut up pieces of a Henry's Block...
www.henryspets.com i cut it in thirds, it gets a wee bit hard over a day and they like to cut teeth on that. you can provide small pieces of apple tree twigs to chew on
an old deer antler piece is ideal it has calcium in it too. i have one antler that has been teethed on by 5 squirrels now. last a long time. I have given almond in a shell for them to teeth on too. Some ;people say don't do that but mine eat their "good" food too, so it is a controversial choice.