Put a few apples in the water bucket. Your horse will love trying to get them and they will last longer too.
Another one is to threde a turnip or swede to a piece of baleing twine and put a hook above the stable door and dangle it from that.
In summer ice cubes of apple juice go down extremely well. Its really funny the first time you give it! What you can do (if you dont mind the mess) is to tie a lolly stick to a piece of baling twine. Put the stick in the top of an apple juice carton dangling (and if its a sealable on clip down the top) and freeze it. After 24 hrs cut away the carton and you have a tasty lick treat to dangle from the hook that you put up for the turnip!
I'm sure there's loads more. Just let your imagination run wild.