Putting in a mini GH right now?... very little except tools, compost and other stuff to keep it dry.... Sowing now in general.... fennel might be worth a go, carrots for late maincrop, parsnips for small uns, all sorts of french and runner beans, sweetcorn (actually that might go in the GH until hatched) if you've got some spare first early seed spuds then they could go in for some salad spuds in September-November... tender herbs like Basil, coriander and cumin....Savoy cabbages and other late winter brassicas need sowing, it's not too late for leeks either...