I would use almost anything without a tough skin, and which will (using common sense) break down in their digestive system. I would also make sure there are no added preservatives, spices (salt, etc.) or things like oil or butter mixed with anything added to an aquarium.
I've heard of lettuce, spinach, peas, potatoes, squash, zuccini, and a few others very commonly used. Other veggies have been named periodically, and work well for those people. Make sure your fish will be able to handle plenty of veggies if you invest and use them often. Most fish will do well with some added to their diet, though.