Somerville: Fast-growing plants?

Although I have a couple of spots I could fill in my garden bed (RIP, cabbages), I'm almost maxed out for space, at this point. Even so, I'm not really feeling done with adding more (more, MORE!) plants. So, the current plan is to add some containers to the garden area, specifically here on this plant stand:

That pot on top will eventually (and hopefully) be a bucket of marigolds.

I'm thinking the containers would be best used for plants with a quick seed-to-harvest time. Maybe I can even get a couple of rounds out of some of them. So, I'm looking for ideas about what would work best here. Got any?

I've poked around online and it's looking like lettuces/greens, radishes, spinach, and scallions are some of the fastest growing vegetable plants. I'm focusing on them because it seems pretty late in the game to be planting anything else (unless I go shopping for seedlings again). But I'm wondering if I could be missing out on other fun options by focusing in on just the zippy growers. What do you, the two people reading this, think?