Herbs?

I am going to plant cantaloupe from seed in a 4' x 4' bed once I dig the potatoes out. It's a really deep bed about 3 feet.

I don't know much about corn. I grew it once many years ago but remember little about the experience except that bugs ate all the corn.

You can plant cantaloupes now and still get a harvest, though you likely won't get any ripe melons until August at the earliest from an early June planting. If you erect a trellis, you could grow several vertically in a 3' x 3' bed. Otherwise, one cantaloupe plant will fill that bed. You still might plant 2 or 3 cantaloupe plants for starters (since pests love them so much) and then after they get growing, thin to 1 or 2. Watch for cucumber beetles because they will spread diseases.

Corn needs a lot of space and I personally wouldn't put it in a 3' x 3' bed. Even if you use the standard spacing of Square Foot Gardening and put 1 plant per square foot, you'd only have 9 plants, and with so few plants, you likely wouldn't even get good pollination, meaning your ears wouldn't be full of kernels. Sweet corn normally is planted from late March to late April in order to get it growing early enough that it can form kernels before extreme heat arrives. Extreme heat can affect corn pollen and prevent the formation of kernels. You could plant corn later this summer for a fall harvest because the late corn would pollinate in the cooler autumn weather, but with only a 3' x 3' bed, you wouldn't get much corn.

It is late to be planting much for the spring garden as most spring garden planting dates have passed, but the attached document lists planting dates for the fall garden, some of which begin around July 1st.

Fall Garden Planning Guide

There are some spring crops that still would produce from a sowing this late, including cucumbers, southern peas (blackeyed peas, crowder peas, pink eye purple hull peas, zipper peas, lady peas or cream peas), sweet potatoes, peppers (hot or sweet), okra or watermelon. With watermelon, the only type that would work in a 3' x 3' space would be the mini watermelons called icebox melons (because they are small enough to easily fit into a refrigerator without hogging all the space). The most well-known icebox melon is called Sugar Baby and there's a smaller, more compact version of it called Bush Sugar Baby. As with the cantaloupes, you'd be able to get 1 plant in x 3' x 3' bed if growing on the ground, or perhaps 3 of them if you grow them on a trellis. Other varieties of icebox melons include Yellow Baby, Yellow Doll, New Orchid and Tiger Baby. There's also a new icebox melon variety that is sized for growing in containers, so it likely would work well in a small bed. It is called Sugar Pot.

You also could plant herbs as Hazel suggested, or some kind of flower.

For informational purposes, since I already linked the Fall Gardening Planning Guide, I'll link the equivalent one for Winter/Spring. The info in it, including planting dates, might help you next year with your planning and planting. Any veggie in it that has a range of planting dates and says "or later" is something you could plant now, though pumpkins would be too big for the space you have, and with summer squash, only 1 plant would fill up the space. Both guides give the standard recommended spacing for each crop and that might help in your garden planning too.

Oklahoma Garden Planning Guide

I'm rethinking the cantaloupe especially since your above comment, Dawn. My neighbor has had a horrible time with it.

Okra could be a good idea, bnissen, or cucumbers. Maybe I'll do one of those too!

I love cantaloupes and other melons and usually get great harvests from them. I generally grow Hale's Best Jumbo which is a great producer here, and usually either Old Israeli or Ha'Ogen or both. Often, when cucumber beetles are bad, I wait until mid-June to plant my cantaloupes, and then I try to plant them as far away from the cucumbers as possible since cucumber beetles are all over the cucumbers by this part of the season. Every now and then, though, the cantaloupes do get a disease and die....and, by cantaloupes, I do mean muskmelons because here in this country we call them cantaloupes but the rest of the world calls them muskmelons. I also grow some true cantaloupes some years, but I didn't this year.

Because melons have a lot of pest issues, I often just grow several kinds of icebox melons. The icebox melons are not as disease-prone.