Tomatoes are fine, but tomatoes leaves are BAD. They are toxic to people and most animals. Never feed your rat licorice, raw sweet potatoe's, Bleu cheese, raw red cabbage, raw Brussle sprouts, raw artichokes, green bananas, green potato skins & Rhubarb, and try not to feed them treats with a lot of sugar too frequently as it can cause diabetes or other health problems.
Rats love eating dried or cooked pasta (especially with tomato sauce), frozen or cooked vegetables, Lettuce, Cheese, cooked Mashed Potato's & lots of other foods, cereal is a good snack to give to your rats, especially healthy ones like Rice Krispies, Cheerios and Corn flakes, Rats also like seeds such as sunflower seeds, pumpkin seeds & peanuts (non salted).(Feed seeds in moderation.)
As a general rule, seeds contain a lot of fat and to many of them in your rats diet could make them overweight.