Onions (from sets), maybe kale. Radishes will be ready to harvest in only 30-40 days, so that's a good, quick variety....

EG

Here is a link that might be useful: EG's garden blog

Potato onions and garlic can be planted in zone 6 'til about the end of October.

If you put up some kind of hoop house (like a clear painter's tarp over electrical-conduit hoops) you can grow things like kale, mache, arugula, spinach... plant pretty thickly because at this point, they won't grow as big due to less sunlight and lower temps. And if in fact they don't reach harvestable size during fall/winter, they will still take off in early spring and you'll get a really early harvest.

Would you consider some perennial herbs like thyme, sage and garlic chives. Sage stays green a long time. Fresh herbs are wonderful.

The hoop house suggestion is terrific.