My horses seem to love toys. I have cones (harvested from along the road where they are obviously homeless cones) balls, squeak toys from the dogs, plastic water bottles and jugs, and my all time favorite, the corrugated PVC pipe from Home Depot. Comes in 10 foot lengths for 10 dollars. I cut them into various sizes and the horses, throw them, chew them, rub them on the fence etc. CJ (buckskin) loves the pipes. Ringo (pinto) loves cones. I put a bunch in his corral and in the morning, he has dropped them each over the fence at Aragorn (black). My mare puts her squeak toys in her water, then pulls them out and they squirt all over. This seems to amuse her.
I also hang chains from the fences and hear them clang around all the time. I have also needed to cover the water tubes with short sections of PVC pipe (the kind that are for joining sections) so the horses don't chew through the tubes. These PVC sections then spin around the water tubes much to the horses amusement hour upon hour. If the wind blows small branches from the trees, I give these to the horses. After eating the leaves, they play with the sticks and chew on them.