How about an air mattress/bed? We have two king-size beds in two bedrooms, but we also have a large rec room downstairs. We have a king-size air mattress in storage for extra company. We've used the air mattress when our bedroom was off limits and the guest bedroom was occupied. They fill quickly, deflate easily and don't take up much storage space when not in use. They aren't that expensive.
I did a quick search at Target's website and came up with a couple of different options/prices.
http://www.target.com/gp/search.html...index%3Dtarget