Ideas on how to make our spare room 'comfy' please!?

We have a spare bedroom in our house - only a small room - which has been unused for the past year. Soon my friend will be coming to stay in our spare room,and there are literally no furnishings. Just curtains,a small chest of drawers,some shelving built into the wall,and an air bed (i know,not exactly the hilton..) what would you suggest i add to make it feel more homely? It's for a boy,by the way.