1. What kind of water buckets do you have or like? I have a big water tub that is half in one horse's run and half in the other.
2. What kind of stall toys do you like? I have never been able to get horses to play with cones, milk jugs (even with grain and little holes for the grain to come out), or jolly balls. For Christmas I like to get them Uncle Jimmys Licky Things.
3. Anything else you can think that I need in each stall? I would recommend getting small mesh hay nets to slow down the horses feed intake and make the hay be spread out over a longer period of time. Also, I assume your stalls will open into an aisle, and so doors leading to outside as well as the doors to the aisle are a good investment. If they are dutch doors, you can leave the upper part open so the horse can stick his head out and have another view, and doors would provide you with an emergency escape door, and you could eventually attach a run or have it open up to a pasture.
4. Should I put a salt block in the stalls? Unless the stalls would be used just for bringing in to give grain, a salt block should be put in the stall. A salt lick wall holder is very space efficient.
Standard Salt Block Holder | Dover Saddlery