If it's not frozen inside, in pots on polystyrene. Watch out for mice.

I agree with Doctorbob, if the temps are above freezing and you have plenty of light (no snow covering your tunnel) then you can plant all manner of vegetables to transplant later. If you have any heating in there you could also plant tomatoes, cucumbers, peppers and the like. Good luck with it and at the price of a packet of seeds it's worth experimenting with one or two seeds of each.