Green leafy vegetable such as lettuce, spinach etc. are easily grown in small spaces.
Cherry tomatoes and bush beans are excellent choices - they would be first on my list. If you like peppers of any kind, you can get tremendous production from a single plant. Green leafy vegetable such as lettuce, spinach etc. are easily grown in small spaces. Consider onion sets, an herb plant here or there, and maybe even a small strawberry patch if you have room. Look up 'intensive gardening' or 'square foot gardening' for more ideas.