It's a bit late in the season really. Tomatoes, cucumbers, etc should be well established by now, but if you can get plants of either, you should still be able to get a crop. You can also grow salad leaves, spring onions, radish etc.
With regard to Winter growing, we tend to 'shut down' from November to January, when we start sowing seeds for the next season. It's extremely expensive to heat a greenhouse in Winter.
BTW what size is your greenhouse mj?
valmarg