Garlic. Ok not quick growing but the cloves from a couple of bulbs will give you enough new bulbs to last all year. Can grow them in pots which frees up the ground for stuff not suitable for pots.
I agree with salads. Pick the right ones and they grow quickly, and keep re growing. We didn't have any issue with slugs, hadn't considered the frog option mentioned above. These were next to a small pond with frogs so maybe helped!