Hey SamS. As day length begins to shorten considerably there is one family of crops that does really well out of this: the brassicas, and specifically salads such mustards, mizuna, rocket/arugula, pak choi etc. These can be sown from late summer/early autumn to crop through most of the winter with a little protection.
To keep the greenhouse warmer for longer, make sure to give it a clean to maximise incoming solar radiation. Some people add insulation to the inside of the greenhouse, but personally I don’t do this as this also cuts out some of the light entering the greenhouse. You could offer temporary insulation in the form of row covers/horticultural fleece on the very coldest nights, but this shouldn’t be necessary if you are growing hardy cool-season crops like the above. Spring/salad onions/scallions do well too, as do winter-hardy varieties of lettuce.
Leave plenty of space between plants - generally at least 23cm/9in in both directions. This will allow more light to reach each individual plant, while offering fewer hiding places for adventurous slugs.