Baseball Tug Toy
I drilled a hole through two baseballs and tied them to a tree branch so that they are just too far out of reach for my dogs to bit onto. They can get them, but it is hard for them to hold on and they slip from their mouths and spin.
Another variation of this is to make a tire swing from an old tire, and from the sides hang the baseballs or another toy so your dogs can play tug of war with each other and the tree.
Editor's note: Thank you for this great tug toy idea! I can just see a dog spending hours trying to get those balls...