Apples in big water bucket.
Those treat balls are good, if you haven't got a fatty!
Suede and various vegetables on ropes.
Treats hidden around bed to get them moving.
Stable mirrors.
If you use a haynet stick carrots in with the hay.
umm...
Stable toys, such as snack-a-balls and likits are good, and stringing up swedes and carrots are good as well. Stable mirrors help keep them occupied and so does hiding carrots round the stable
Ditto what everyone esle said. My girl is due to have 2 months box rest soon (2 weeks time) so I have bought a stall guard so her door can be open. You could also get a decahedron and put nuts in it for your horse to kick around the stable.
Before you go out and spend money - does your horse really like/want/need entertainment? My yearling had to have 6 weeks box rest and was used to being out 24/7 so I really thought she would need "entertaining". I went out and bought one of those scented apple things and 3 LikiIts (cost me a total of £45!) . . . . and did she even LOOK at any of them - nope!