The traditional futon is quite comfortable.  It is a very different feel thaan anything I can think of.  At first, your guest might be a little taken aback because it does seem very thin, but when they lay down and get to sleep they'll realize that it's much more comfy than they thought....it also makes your back feel amazing.  You could also through some thick comforters on top of the futon for some extra padding.

Therm A Rest, Seattle, USA makes a very fine inflatable camping mattress (seems like impregnated canvas).  We've had ours for 15 years and use them for emergency stay-overs as well as the great outdoors.  Alternatively, have your guests bring their own - saves on decisions, storage space and responsibility for comfort.  Now, what to do about extra bathroom capacity...

My friends have various sleeping furniture that works very well.  I don't know if its less hazardous than pvc since i know that was not their objective but its still pretty neat.  They have a loveseat/big chair that folds out like a sleeper sofa and they have a large ottoman that folds out to be single bed. Both are pretty comfy and you would never know just looking at the furniture.  Prior to these pieces they had what was referred to as the 'chair bed'.  It was a chair that had a z-folded mattress like the above mentioned japanese mattress.  It was quite old but I do have to say it was really uncomfortable as a chair and as a bed.  But I think it was just realy really old.  The places that they got the above mentioned furniture was from Crate and Barrel and Costco.  I have seen z-folded foam foldout sleeper sofas and chairs in the LL Bean catalog. Don't know if they are chemical free though.

Be careful with the latex foam. So people are allergic to latex.

I spent 4 years sleeping on Japanese futons. Most Japanese futons consist of a cloth covered z-folded foam piece on the bottom with the cotton shikibuton placed on top of that. The shikibuton was then covered with a sheet and blankets or heavier quilt thrown on the top to sleep under depending on the season. Very comfortable and it all fit nicely in the closet except during my bachelor days when it hardly got put up at all.

We have three futons - two that live on the floor and one futon-couch (the fu-couch, as I call it). Thus we have lots of nice lounging spots and plenty of places to put guests. They're ideal in a house that has the shoes-off rule because they don't accumulate much cruft. We keep sheets on ours and shake them out once a week.

We do have a vinyl matress purchased years ago for those folks who are so sensitive to cat-ness that they have to have a pristine sleep surface. We've slept on it when car camping and when we were moving into our new place, and they have become much more comfortable recently - the state of the art has advanced significantly in air matresses. Too bad they're vile and evil.

One thing largely unrelated to the above, but still worth knowing:

When a hostel has air mattresses instead of the ordinary sort, it is sometimes an indication that they are infested with bedbugs or other vermin. Any time I see them, I make sure to check around the beds for bugs or blood speckles, as well as poll people who have stayed a few nights about any bites or other unpleasant effects.

I like those camping cots with the canvas (think organic canvas!) and the X-folding legs. Some of them also fold in two after you collapse the legs and then they are about the size of a card table - easy to store. You can put a single foam pad on top if you need to and sleep up off the floor in comfort.

I also have a double bed sized down comforter that, folded over, makes a great overnight mattress. My grandson loves it. So cuddly . . .

install some hooks in the ceiling.  When sleepy, attach hammocks.  Enjoy.