I like all the toy suggestions, but I have one that works wonders when it rains or I am studying. We call it the “red eye of death” lol. Actually, it is a laser pointer. Bully will chase the red dot until he drops. It is great exercise and he will pick up the laser pointer and beg me to turn it on. I think it provides a lot of mental stimulation. My dog went through a seperation anxiety when I first rescued him. I left him in the laundry room and came home to find he had chewed a hole through a wooden door. I was horrified and we went to the vet. The vet and the trainer helped us solve this anxiety. I think you see it alot in rescued dogs.