Mustard greens are another class of brassica! (There are lots. *g*) It looks like they do *better* in cooler weather, but they, and arugula and the others, *can* take some heat, and they do mature really fast. (You can cut the leaves off, leaving the roots, and they'll come back a couple of times before the plants are finished.)

Returning to the lettuce question -- loose-leaf varieties might do you better than the heading varieties. Um, oak leaf is one that springs to mind? They can be picked/cut (same as above, "cut and come again") earlier, before a heading lettuce would have, well, headed. Plenty of water is going to be key, mind you, and if they start to bolt, they're going to be bitter.

Oh, passing on a piece of info I just got from a very successful tomato grower: if the seedlings get very tall, just plant them deeper when you pot them up or plant them out -- you can bury the extra stem/leaves. It encourages them to put out more side roots, and makes for a stronger plant.

Zucchini would love to grow for you (note: they take a *lot* of water), but they are big; one would probably take up your whole bed. But you could grow one in a container, provided you make sure it doesn't dry out, or just consider it going forward.