Go to a lumber yard or home improvement center and have them cut a piece of plywood slightly smaller than your hide a bed mattress. Remove the mattress from your bed when it is fully open. Lay the plywood over the frame and place the mattress on top of the plywood. This will prevent the bar from sticking up through the mattress.
Add a foam egg-crate mattress pad on top of the mattress. Even the thinnest mattress will feel more comfy with two inches of squashy foam on top.
Cover the egg-crate pad with a woven mattress pad. These pads simulate the cushioning in a pillow-top mattress, so they add just that much more comfort to the mix.
Make up the bed with good linens. Use all-cotton sheets with a high thread count for a satiny smooth feel. Add a soft comforter and lots of big pillows for nighttime reading. Great linens go a long way toward your guest perceiving your hide a bed as a place of comfort.