Rats will play with all sorts of toys. Cat toys like the ones with feathers on them are fun for them to chase, and they will play with ping pong balls, balls with bells in them and other objects that roll. They also enjoy bird toys with bells or parts they can chew and dog chew bones. Your best bet, though, is toys that you find around the house or make yourself. You just can't beat a toilet tissue roll with some paper left on it for tearing up and playing with. Hide treats in an empty tube, fold the ends and watch them work to get to those treats.
Water
Some rats like to play or swim in water especially if they have been exposed to it since they were young. A paint pan makes a great pool because it has a shallow end so your rats can ease into the water. If your rat likes the water, be sure to give him plenty of opportunities to play in it as this can make things easier at bathtime. You can even put bits of frozen vegetables in the pool for him to "dive" for and eat. This is big fun for them and especially good on a warm day. Dry them when playtime is over with a soft towel.