YAY for greenhouses! (Those on the allotment thread may remember that I actually had an asthma attack with excitement when my first ever greenhouse arrived earlier this year). I'm so glad you've kept it! This could be the start of a beautiful new friendship with your garden.
3m x 2m is an awesome size - how wonderful to have so much space! There are all kinds of things you can do in there, some that are quite permanent (e.g. growing a grape vine), others that are more annual (e.g. growing tomatoes, aubergines, peppers, cucumbers, all hot weather plants that get sown in late winter/early spring).
And then there are about a billion different things you could grow from seed and then plant out later in your garden - not just veg, but flowers, shrubs etc. etc. etc. I'm finding that it is quite a difficult and skilled thing to grow from seed - I definitely don't have anything like a 100% success rate - but it is tremendously rewarding to see a great big plant that has sprouted from a tiny seed that you sowed.
This time of year is quite late for sowing a lot of seeds - spring tends to be the peak moment. However, there are things that can 'go on' now for next year.
Flowers: violas for next spring from nowish, and from late Sept/Oct you can get on aquilegia, hollyhocks, lupins, salvias, verbascum, helenium, hardy geraniums, euphorbias, dieramas, even sweet peas...
Veg: it's not too late for winter lettuces and other hardy winter leaf crops to go on now, swiss chard, kohl rabi, perpetual spinach, and your winter cabbages and caulis for maturing early next spring. A lot of people sow these directly into the ground, but mine all get eaten by slugs if I do that, so I start everything in the greenhouse.