One of these years I'll put in one of those sun porches (they have some great looking ones in the hardware stores, and even Garden Ridge was promoting a very attractive glass-roofed and sided porch enclosure). It will become the defacto greenhouse when that happens. If it is possible to keep it cool enough in summer, the greenhouse would be more useful for summer plants than winter plants.

FWIW, I'm usually able to plant leafy greens by direct seed every weekend thru october one row at at a time to get a constant November salad. Spinach, mache, mesclun all do well in the cool climate and can tolerate more sun since it's cooled off. Oh, and beets too if you're into that :?.

Carrots can if covered on cool days to keep the ground a little warmer can go almost year round I have found.