what to grow in a cold frame?

It would be classed as an unheated greenhouse 

You can plant seeds that are hardy in it for a bit of start, like radishes, and leeks in trays,  but I would avoid giving cabbages or similar plants any protection or they get very leggy. Treat these tough   

Lettuces and sald leaves will do well right now, but don't let them burn in the sun during the day -- easy done 

Neither cabbages or onions sets need any heat or protection tbh

Trouble is, they then grow too quickly and topple over, making spindly plants that never recover.

Some people do start off their onion sets in a cold greenhouse, in pots and then transplant them out, but I never have bothered and they grow just fine.

Save your indoor space for those what needs it is my advice