Add texture as well as color. The room will feel more alive when all the surfaces are not flat.
Light (and / or lighting) is the cheapest furnishing, which takes up the least amount of space and can work wonders in a small room.
Pick a large scale print on a fabric or rug and repeat its colors in smaller patterns in the room. Good design includes repetition.
A small space needs furniture that can serve more than one purpose. Beds, with drawers underneath, tables that can serve as a desk or for dining, comfortable chairs for sitting that can be pulled up to the dining table at dinner time.