building soil with cover crop/animal rotation

[mostaza]

I have a large space outside my garden fence for which I have only limited energy to devote. I chopped down the thin grass/weeds and have spread some old hay that I wasn't going to use. the area gets full sun and will be plenty moist all summer.
THE QUESTION: What could I plant right now that would bring good nutrients to the soil and be ready to till in for fall planting(or do you have a better idea?)

[bullthistle]

Legumes.

[!potatoes!]

yup. vetch, maybe peas, clover, cowpeas...