I reckon something taller at the back like the dwarf beans Jenny suggested. Or how about peppers, chillies or augergines? I saw some fab ones growing in cold frames at Wisley this summer. They had pinched them out to keep the plants stocky and they were dripping with fruit. You could then use the lower bits at the front for salads and stuff.
I've tried melons before, but found them better grown up supports in the greenhouse than sprawling over the ground - they take up as much room as squash plants, so you may not get much return for all the cold frame space they eat up.
Sue