The mice love their running wheels the best. Since they are in aquariums, I had a difficult time finding something that would utilize the vertical space until I came across a shelf made to hold an aquarium pump. It hooks to the rim of the aquarium and hangs down about 4 inches and gives about a 3 inch by 5 inch flat surface to sit on. I then use bird ladders for the mice to climb to reach the shelves. (at least 2 shelves per cage) They love this extra space. If you have wire cages, a shelf would be a lot easier to attatch and it can be as large as you'd like. I bet the rats could use bird ladders for climbing.