In addition to "rat" or "lab" blocks (basically square kibble), you can supplement your rats diet with fresh fruits, vegetables, (my rats are happy to share whatever I'm having as a vegetable side-dish for dinner) and non-sweetened cereals such as Cheerios. Rats can eat seeds and grains, but shouldn't have them as the bulk of their diet. They can have occasional treats like a tiny piece of chocolate (it's not toxic to rats) or a yogurt drop. Also make sure your rats have fresh water at all times.
Here are some links that provide a lot of good suggestions for feeding your rat as well as receipes for preparing a homemade diet: