What to plant following what was planted depends. Cool season plants sucha as peas can be followed by tomatoes, peppers, etc. Cool, and long season plants such as the cabbage family will be there for the full growing season most places. Beans could follow lettuces which could then follow the beans.
Lay out your garden and look at what you want to grow and which need to be in early and will fade out in heat. Then look at what you want to grow that should be planted during really warm weatehr and see where it fits in the total plan.

A densely-planted mix of chickling vetch, buckwheat and cowpeas might work.
I'm reading from your post that you are planning to grow non-food plants in that area over the summer. You'll generally get the most nitrogen-fixation from legumes before they flower.
Where are you?

Hi Feijoas, I live in eastern KS, close to Kansas City.

Buckwheat will do very well in the heat and bees love it. Another one I've been wanting to try is sudangrass. Usually though my garden is FULL doing the warm months and it seems difficult to work into a rotation here in eastern Colorado.

You can use New Zealand white clover in with your crops as a living mulch and improve your soil at the same time. I do that with some of the pink clovers, too.

I can't seem to get the timing to work on some of the other cover crops unless I forego Spring planting in some areas.

My favorite summer plantings are okra, cowpea, squash, etc.
It all depends on what you want for your overall rotation and what immediate effect you want.
Many covers will be hard to get rid of once planted until they bloom next year or a hard enough freeze occurs.
Be careful.

Here is a link that might be useful: Crop Rotation