It's important that rats exercise in order to keep healthy. You can help them by putting lots of things to climb on in the cage, and having everything spread apart, like the food in one corner, the water in another, and the bed in a third. Rope climbing structures are always good, and I've heard some nice stuff about wheels, but never tried them. Some things to be ware of is that while a lot of rats do like wheels, it's rare for rats to use them as much as mice or hamsters, and you have to get a really big wheel or they can hurt their spines. Another thing I've heard is that rats that run on wheels have permanently bent tails, and that you should avoid wheels made of wire. Stairs are great for exercising rats, but you need to make sure that they can't escape off either end. If you run up and down the stairs, some rats may chase you, but this is also very tiring for the human, not just the rat, and you need to be careful not to step on them while you're running. If your rats know their names, you can also call them up and down the stairs, which is a lot less dangerous, but not quite so much fun. I like to do a little of both when I play with my rats.