Radishes are the thing. They germinate fast and grow fast, and they're fun to pull up and eat.

You could also do peas. Again, they germinate fast. Might not be quite enough time for them to get to harvest size, but the pea greens themselves are edible, and of course the immature pods. And they are easy to plant.
-O

Yes, try planting some things for baby greens--I recently planted lettuce, spinach, kale, mustard greens, and collards. I know here in zone 5 they probably won't get bigger than a couple of inches before a freeze, but they are still great for salads. I would have also planted beets for baby beet greens if I had had the seeds. (These vegetables take about 25 days to be baby green size.) Being in zone 8 you probably still have at least a month or two for growing.

Leeks Onions Radishes Lettuce Spinach Collards Garlic Kale Turnips Brussel Sprouts Mustard Beets Carrots are some at the top of my head. Look for things that do well in cold or 45 to 60 days to mature.