However, I don't recommend wire wheels, as these can catch and injure legs or tails. You can sometimes find solid metal wheels, or there is a solid plastic wheel called the Wodent Wheel that is wonderful. It comes in 3 sizes, Junior, for hamsters and mice, Senior for small rats, and Wobust for larger rats. Some rats enjoy small toy balls, especially those with noise-makers in them, and other small items that they can carry or push around the cage. Look in pet stores for cat toys.
Balls made of rope also work well but avoid toys made of soft rubber.
Some rats like stuffed animals and carry them around and sleep with them, while other rats just chew them up. You should offer some chew toys for your rats. Some rats like rawhide chews, and most like wood and cardboard chew toys. All bones, even chicken bones, are safe for rats. There are a variety of food toys that can make rats work for their food and keep them occupied.