You already know that a futon can work as a serviceable sofa as well as a bed, but there are other types of furnishings that do double-duty, too. Some footstools or seating cubes open to provide handy storage. Place a serving tray on top, and you have a makeshift coffee table, foot rest and linen bin in one inexpensive piece.
Drop leaf tables are great multifunction space savers, too. Fully opened, they can seat four to six, but remove one leaf and the table will be perfect for intimate dining à deux. Drop the other leaf, and you'll end up with a narrow table suitable for a hallway or entrance. Best of all, some of these dining pieces have onboard storage for folding chairs.
When you live in a studio apartment, a furniture piece that takes its tiny footprint and flexible functionality seriously is a good friend to have around.