Rats can't digest rabbit food which is alfalfa.

Give it any kind of seeds or grains, dry oatmeal, stale non white bread, lunch meat, tuna, cooked egg, fresh or frozen vegetables, rice, dry or cooked pasta, dry cereal (unsweetened is better)... most human food can be eaten by rats.

Dry dog food is good, or dry cat food, though cat food is very high in fat and protein.

Avoid corn, onions, dry beans or peas.