herbs give you a lot of bang for your buck. You can start harvesting almost immediately, you don't need large pots (maybe one gallon or so) and you can continue to harvest until a frost. I have done parsley, cilantro, basil (many types!), mint, rosemary, thyme, oregano, chamomile, and several others in pots.
If you have enough space for a tomato plant or two (they will get about 4-5 ft. tall and about 2-3 ft. wide, you should get grape and cherry tomatoes. Most of them bear a ton of fruit with very little effort. One of each will keep your salads full of tomatoes for months! Honestly, it really depends on what you like to cook with. I have grown peppers, onions, horseradish, green beans, peas and carrots in pots and they usually turn out great.