I was over at the Vegetable Forum and read that peas can be sown at this date in Zone 5.

Lettuce, swiss chard, kale, beets, or mustard should all work .. lettuce might be tricky to germinate right now as the soil is very warm but the others should come up fine assuming you're not dealing with extreme heat.

Peas are doable too but also tricky to germinate in the heat and a bit of a crap shoot as, while they can tolerate some light frost, they're not too happy about solid killing frosts that can occur up north as early as September (here in Alberta it's not unheard of to have a frost like that by Sept 15).

On that note, I sowed some peas in a self watering container on Friday .. it's sitting in the shade and getting mid-afternoon daily doses of cold water until I see plants. Hope to be harvesting by late September with a little luck. :)

What was suggested above, plus radishes (sow a month from now), baby bok choy (now and later, staggered) and spinach (now). An early carrot variety might work.
If you can use hoops with poly and/or row covers, you could probably extend the harvest season into November or December, but expect growth to stop by late Oct.

Thanks for the suggestions. There is now some fresh growth in the garden, as the beets, radishes, spinach, and chard have all started to come along quite nicely!!!