What can be in an unheated greenhouse?

Ideally you would put them on staging in the greenhouse as, on the floor, slugs can be a big problem with little seedlings. I've got onions. leeks, brassicas, beetroots and lettuce out.we've had several frosts and I've accidentally left the door open a few nights and they are all coming along nicely.
The onions and leek are only like blades of grass in seed trays but I'd like to get them planted out. I did this last year and got a bumper crop of both with not too much effort (sow the seeds, plant out, hoe a couple of times then harvest).