Cucumbers

Cukes are very cold sensitive so if you're sure the cold frame won't be too cold, you could put them in there. Some cukes are bush variety, most are climbers, so it helps to know what you have. I prefer to let mine climb (my Munty frame) to save space. In a cold frame, they can scramble but you couldn't fit too many in there or you risk mold from cramped growth. If you've not enough space, you could plant them outside against a brick wall for warmth as they're a tropical plant.