kale comes to mind- ours usually keeps producing until the temps drop into the teens. Cornsalad is very hardy. There may still be time for a crop of radishes (try them in any recipe that calls for turnips!)
I read that with a lot of the hardier plants, if they are frozen, leave them alone- often, once the sun hits them the leaves thaw out and the plant is undamaged.
Turnips and mustard comes to mind. Mustard is very cold tolerant, and turnips get better as the days get colder. Maybe broccoli or cabbage, depending on how much time you have to acquire plants and grow. Then you could try short season peas or (if you have time) bush beans. Then spinach as it loves cold weather...just a few ideas.