I have already begun to sow bok choy seed for the fall. That will continue for about another month.
It won't just be all bok choy, all the time. There is komatsuna seed to get into the ground also

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This can be varied further, of course. It is just what I have been successful doing and reflects my tastes. I haven't really cared for turnips but it must have been over 30 years since I've grown them and I should really go back and find out if I like the greens. Bok choy is in the same family with turnips and that's a fairly broad family, there are other things.
Komatsuna is in the mustard family, as best as I understand, and an even broader family. You may be able to grow a quick-maturing cabbage from transplants or broccoli, I really don't know since I've never done this. There are lots of late-maturing cabbage out in the garden that were set out in the spring. All of my early cabbage has been harvested.
So, that's the idea - quick-maturing greens. I don't know why I don't sow spinach seed but I don't have a great deal of confidence in it. Spinach really likes to bolt to seed here and might just rush into that stage. One thing, lots of people plant spinach to over-winter but we aren't quite to that point in zone 5.
I also have some lettuce starts that will be transplanted out in a couple of weeks. They should do fine but direct-seeding in the garden hasn't worked very well and I haven't had much luck with radish at this time of year. Other gardeners may have some ideas about carrots.
Steve