Off-Gridder
Trundle bed - it's on wheels and rolls under your regular bed.

Lost Cause
Try a hammock. Unconventional maybe, but I slept on one for two years and if hung correctly are VERY comfortable. I just put a couple of eye bolts in the wall studs and put the hammock up only when going to sleep. It is very easy to store and you wouldn't likely have to change anything about your current room setup. Several low cost options are available via the internet but I would stay away from most spreader-bar type models. I had a Mayan woven style hammock (bigger is much more comfortable), but non woven hammocks are just as comfortable. or for information if you wanted to make a non-woven hammock yourself.

Margaret C
Futons can be very comfy

PattyAnn
How about sleeping bags? You can get some very nice plush ones, and they roll right up for storage. I would sure prefer that over those metal cots. We have used ours often when visiting someone, and it's been quite comfortable. Just a thought.