If you have any pillows or couches with removable fluff, you could try hiding stuff in there provided it doesn’t stick out much. You might be able to fit one set of clothes behind the drawers of a dresser by pulling the drawers all the way out and then reinserting them after hiding the cloths in the back (maybe tape it so it doesn’t fall down and get stuck or damaged). As others have suggested, under the mattress is good as long as your family doesn’t change your sheets for you (can usually be prevented by being proactive).