Popped popcorn if you have an airpopper. There are also little cobs you can buy in the small animal section that yield very soft/tender popcorn that the rats will love.
We used to give ours a variety of nuts and seeds.
You want to be careful with anything sweet. Rats can develop a sweet tooth very easily/quickly. And this includes fruits. I've seen a few rats practically starve themselves in order to get their sugar fix foods.