Are you being realistic about how much room the squash plants will take up? I always underestimate. What about kale? It can get pretty bushy. I grew lacinato last year. I planted gilfeather turnips in mid-June last year and they did fine.

Radishes are quick, you could harvest them before the squash takes over the space.

Lettuces could be picked from between the squash leaves.

Radishes and lettuce won't do well seeded in hot weather, if the OP is having it now. Is there enough space for a little patch of corn?

Maybe bush cukes or an early bush melon?

The heat and current drought we are having isn't good for lettuce...a few of my squash have reached full spread and I've planted them before, so I know how big they get. I have a lot of cukes already and I live in sweet corn country...but maybe some radishes! Also, I don't have a lot of plant shops around me that carry odd/non traditional plants, so I don't have much selection. Kale might be good, if I can find a kale plant to buy! Thanks for the advice!!!

Beans would be good to plant about now.

You could do some beets. I would like to try some beets myself! My neighbors have beets growing in their garden now and also radishes. Maybe you could do horseradish if you like that sort of thing!

Is there room for a teepee of climbing beans? Small foot print but lots of vertical growth. You could direct sow half a dozen beans and they'd be up in no time if it's warm where you are.