Hey! This is pretty neat. Do you have an old swing that your kids have grown out of? Use it as a mini green house! I know the above photo via The Haby Godess is a chicken coup but just replace the mesh wire with plastic and you get the same thing. The swing makes a great frame! Tall enough to stand up in and if you propped the swing legs on cinder blocks it could be even taller.
Check out this 8' x 12' x 8' swing set greenhouse via the Easy Gardener. The Greenhouse is made out of a old wood A-Frame swing set. Made with 6 mil plastic. The owner said that the greenhouse heated up well with little sun EVEN on 25° days and for the real cold days and nights he used an older forced air Kerosene Heater with a thermostat set at 70°. The old hay bales around the green house were to seal the bottom of the plastic to the ground.