Per Miss Manners, all plumtreeblossom has to say is, "I'm afraid can't sleep on the floor." No reason is necessary. The whole "I can't . . ." is a great face saver. "So sorry I can't come to dinner on Friday," could mean you have other plans, or that you can't stand the thought of an entire meal spent in the other person's company. But "I can't come to dinner" sounds a lot better than desperately struggling to find a plausible excuse, which could then be countermanded:

"So sorry, I'd love to come to dinner, but . . . I have a friend from out of town"
"Great! The more then merrier, bring her along"

or

"So sorry, I'm afraid I just started the Atkins diet and so eating out is problematic."
"No problem! I've been dying to try out some of the recipes in "Better Cooking with Suet and Lard."

or

"Oh, I"m about ready to drop at any minute, and I'd hate to spoil your dinner by going into labor."
"Oh, don't worry about that. I"ve been reading Obstetrics for Dummies and it would be a good chance to apply my new skills."