Welcome to A4A aligator. Garlic and onions, overwintering peas and broad beans, oriental leaves (pak choi etc), radishes (very quick growing for kids but they might not like to eat them!), or failing all else a green manure like phacelia or poached egg plant which you can dig in in the Spring. It's nearly time for putting in fruit bushes/trees, so if you're planning for any, time to go shopping!