I love tennis balls and I do not play tennis. My favorite use for them is to use them for massage. I can rub my foot over them to help with plantar fasciitis or just for fun. I also place one on the muscles between the spine and shoulder blade and lean against a wall.Then, doing mini-squats rolls out the muscle. For more intense pressure on the back, I place 2 in a sock and lay down on them. The gap in the middle keeps pressure off the spine while the muscles on either side relax into the pressure. Racquetballs can also work. Golf balls are okay for the foot massage.
Other uses for tennis balls: