I use my poorly insulated front porch to a similar extent. It's a ~10'x15' room. With a decent door (might have to change it more to an 'exterior' door at some point) and new double pane with argon gas, blah blah blah window between it any my living room (see below for living room side pic) The room gets the sun a good portion of the day, so during early winter, it stays closed up, and when i get home from work, I'll open the front door, and let the heat seep into the living room. It'll usually raise the temp a few degrees. I never really thought about how to make it HOTTER in there.
During the summer, this room stays sealed up, except that I'll leave the windows open, so it's only as hot as the air outside, no hotter (well kinda).
