In warm(er) weather, freeze a bucket of water (or “hay soup”?) with chunks of apple, carrot, orange, cucumber, or whatever else your horse likes and pop it out of the bucket so he can get at it. If that’s too close to “food” for the other horse, maybe just large chunks of ice.
A yoga ball was a big hit for my young horse. Once that was destroyed I switch to a soccer ball but he didn’t like that nearly as much.
I’ve seen horses play tug of war over a rope. I wouldn’t have believed it if I hadn’t caught it on camera.
Agree that you have to rotate toys with the youngsters who seem to be part Labrador puppy. A few other things besides those already mentioned are ground poles, a portable mounting block and an empty much bucket. Don’t leave the muck fork or the poop in the muck tub in the corner of the arena if you use the arena for a playground. Ask me how I know! And yeah, don’t leave a lunge whip either unless you want him to try to lunge his buddy.
We had a STB racehorse that adored rubber chickens. Fortunately the squeaker didn’t last too long with his enjoyment.