Peas-peas-and more peas! Lol! Mine also eat letuce and grass. They have eaten a little bit of tomatoes just the skin not the seed. Not sure if the seeds would hurt them.
But they LOVE LOVE LOVE peas! When they are really young, probably a week or so I mash the peas up and add a little bit of water to make kind of a pea soup. After they are a little older you can give them whole peas. Its especially cute when you put some in their bath and they go diving for them.