Buckwheat! I hadn't thought about that.
I planted buckwheat in some of my garden beds after I'd harvested the onions or garlic in them. It has a lot to recommend itself:
Honey bees love it.
Planting the seed to bloom in ... 6 weeks? Maybe less.
Crowded out weeds.
Cut it down, let it return to the soil.
Frost kills it, so you don't need to worry about the plants overwintering and being a problem like, ahem, alfalfa.
It does not fix nitrogen. I'll see how many seeds sprout from last season.
Did I mention that it blooms quickly and the honey bees love it? I'll be planting it again this year in garden areas that need some filling in.