Try heirloom tomatoes - see if you can get plants with blossoms on 'em already those are SOOO much better than store bought.I'm also doing radishes from seed for the summer - they need shade but are growing really fast.
~plantgirl of Plantgirl's Square Foot Garden

Plantgirl:

Thanks a lot for your reply. I should be more specific: I'm already growing 10 indeterminate tomatoes, 9 of them heirloom varieties. And I agree, absolutely nothing can beat a garden fresh tomato. I'm also growing bush beans, runner beans, chard, carrots, peas and fingerling potatoes.

What I'm really asking is what do people generally do with the spaces freed up when their spring crops are done? In the past I've tried to continue to grow lettuce but it just doesn't do well in the heat. I'm assuming that by early july it's probably too late in zone 6 to add heat loving crops like peppers or tomatoes, but kind of early to start planting fall/winter crops such as kale.

Last year (after the peas were done) I used 2 squares for pole beans, 4 plants at each corner supported by 4 bamboo poles. In the center of the tee-pee I planted 4 heads of romane lettuce.

Those plants produced all summer long without bolting.

This year, after my spring veggies are done (for me mostly beets and kohlrabi -- both planted late), I plan to leave the squares free for 2-3 weeks, then plant my fall turnips, parsnips and other fall root veggies.