It'll be a while before you start squash away, but tomatoes, aubergines, peppers, chillis etc can all get going now if you have somewhere to keep them out of the cold once they're up. Also a good time to module-start leeks, early beetroot, a few early lerttuces, all sorts of onions from seed, spring sown garlic and shallots can go in modules too. If you push a toilet-roll tube into each cell of a 24-cell tray then you've got a cheap version of a rootrainer, good for module starting carrots and parsnips. You can kick off celeriac now too, and seed-grown strawberries, you might try a few summer cabbage though it's a bit early, all-year round caulis fmight be worth a few modules too...