We sometimes find that we are in need of an extra bed. If this is a rare occurrence in your household, then there are plenty of inflatable type matresses that will suit your needs. They have the advantage of being easily stored away in a closet. If, however guests or family members stay overnight on a semi-regular basis, then a trundle bed may be in order. They are practical, but have the advantage of being attractive pieces of furniture.
White trundle bed:
White trundle bed pictured: Fashion Bed Group Casey Daybed with Trundle
Also of interest: Day Bed with Trundle Mission Style in White Finish / Coaster Wood Daybed With Trundle in White Finish / Trundle Bed In Glossy White Finish, all by Coaster Home Furnishings