I have a couple of stable toys for my neds. One is an Eggbutt ball, which is a large inflatable rubber ball, with a handle for the horse to grab hold of. It cost about £20. My horse was pretty keen on it initially, but soon lost interest.
The other thing is a Boredom Breaker. This is a ball with a treat holder positioned above it. You hang the whole thing up, at least 3 feet away from any wall. You can put apples, pieces of carrot or swedes on the treat bar, and it takes a while for them to get it off. You can also buy a purpose made treat called a Lickit, to put on the treat bar. This is like a large boiled sweet, made from cereals and molasses, and lasts longer. They come in different flavors. The Boredom Breaker costs about £17, and a Lickit costs £2. My horses really like this. I have hung it over the partition between their stables, so they can play ping pong with it. This seems to work OK as they get on well.
I also try to turn my horses out as much as possible to avoid boredom, although I appreciate this is just not possible for some people.
Hope this has been of some use,
Marie-Ann.