I don't have the answer, but google 'succession planting' - example:
http://www.seedsofchange.com/enewsletter/issue_68/successionplanting.aspx

The link given is totally useless for me because "the rules are different here" in Miami, Florida. Dates given are for northern climates and don't apply here. We rarely have frosts and even rarer freeze at all. Most everything dies in summer or gets eaten alive by insects.

I said the link is an example...you can look it up for the area where you live...

We are talking about in containers, yes? Radish, brussel sprouts and green beans are not going to work in the summer. And that's the Houston summer, Miami is probably worse, no?

I'm always on the lookout for veggies that will prosper in the heat. Most of these were in the ground, but I've seen them work in containers if they're large enough and don't overheat the roots (a common problem in pots). Here's what I can think of off the top of my head:

Leaf Amaranth - Delicious and incredibly easy to grow. Some varieties are also beautiful.

Swiss Chard and Kale - may test less than ideal, but won't bolt or become inedible like most brassica

Malabar Spinach - *beautiful* and very tasty, could care less about the heat. Easy to find even in the US.

Chaya - tree spinach, may take some work to find

Okra - the epitome of a sun worshiper. In last years century-record setting summer in Texas, Okra went gangbusters.

Southern peas - also called cowpeas, blackeyed peas

Yardlong beans - Asian green beans

Eggplant - loves the sun, though we're at the tail end of planting time

Gourds, melons, cucurbits - There are literally hundreds of options for good summer crops. Traditional cucumbers will become very bitter, but there are variety of Cucumis melo cultivars that taste like cucumber and never become bitter. These include the italian cucumber melon and the Armenian cucumber. Other asian cucurbits, like the "ivy gourd" or "bitter melon" are other great vegetables that absolutely adore the heat and have very few pest problems in the US. As for sweet melons, there are more than you can imagine. I'm growing Sugarbaby watermelons and American cantaloupes this year.

Sunchokes - a root crop with beautiful flowers, fit for anyone's front yard.

Sweet potatoes - start them now if you want a crop before winter.

A lot of good ideas. Unfortunately, None I like except sweet potatoes. Will regular potatoes work in 18 gallon containers?