I've got alfalfa planted. It fixes nitrogen and adds humus when tilled in.

Peas and beans are a good choice, if you have left over seed. They are legumes, and will fix nitrogen too. I usually grow buckwheat as a fast green manure crop. In Vermont, I can easily get two crops of buckwheat on a fallow garden, or one crop after my early veggies are done.

Winter rye is good too, that's what I've used this past year.

I use buckwheat for the nitrogen, fast green material, and ease of incorporation. We only have a small rototiller, and hand labor. I have been amazed at how much a single application can do for my soil!

I keep a spare bag or two of maple and filbert leaves on tap at all times and I give my garlic bed a generous heap of leaves in the early fall before I ever plant. I replenish as needed to keep a few inches of mulch on until around June when the tops start to dry off.