Finish the floor with carpet or a heated floor. The lower level of bi-level homes are underground and tend to be cooler than other parts of the house. Carpet will keep your feet warm, but you can also install heated tiles or add heated pads under laminate to give ambient heat to the room.
Paint the walls and ceilings with a light color. This brightens the room and makes it look bigger. If the room is spacious, you can use darker colors on the walls, but still use a light color on the ceiling. Light ceilings look higher than dark ceilings.
Add baseboard trim to the room, but avoid using wide crown molding or trim around the ceiling. This gives small spaces a boxed-in look. Use smaller trim to neatly finish the room; 1 to 1 1/2-inch trim works well.
Add overhead lighting, such as pocket lights, and wall sconces to the room. The lower windows in bi-level homes are smaller than the upper windows, so the room won't get as much natural light. Lights that fill in the room's shadows will create an open look to the room. If you don't want to install sconces, use floor and table lamps for additional lighting.