Spinach and lettuce are probably your best bet right now. Pea plants are not frost sensitive but the flowers are, not usually grown as a fall crop because of this. Carrots generally take a long time and are sown mid summer for a fall crop. Bok choy might work out for you, too, radishes as well.

Almost September is pretty late for fall sowing in northern Colorado. Most crops grown for fall harvest are planted by mid July really. Cover crops/green manure are always an option though, to get the soil ready for next spring.

Also beets, swiss chard and maybe scallions. I'm also trying fava beans this fall. I tried it as a spring crop but it didn't like the hail at all.