Broccoli family has some greens that will reseed well. You are a bit colder zone than me but I think they would.
Red mustard and some of the Asian greens come back for me. I would pick one, though, as they might cross pollinate if you do more. Red mustard can be pretty, with dark red big leaves and then bright yellow flowers.
I'm pretty sure tomatillos are going to volunteer. Next year I may just look for volunteers, and have them coming on as tomatoes are winding down.
