One piece of advice from my father, a lifelong gardener:'stick to the stuff that's expensive to buy'.
He wouldn't grow carrots, for example. By the time you'd dug them up, washed them, prepped them, it was easier just to go down the shop and buy a pound.
In a small greenhouse, you're best off growing compact capsicums, chillis and maybe aubergines. You can grow tomatoes outside, especially the cherry varieties and ones like Gardener's Delight. Start them in the greenhouse, prick out to 9cm pots, plant out when frost danger is over.
Stuff like beetroot, parsnip and carrots are just