Ceiling light fixtures, fans, or even skylights can add interest. Don't be afraid to consider painting your ceiling. Start a shade or two lighter than the chip you like since the color intensifies when you put it on a larger area. Wall materials and their finishes are third on the list. Wall options tend to be the most overwhelming, but if you've already made your decisions on the flooring and ceiling, it should be easier to narrow your choices.
Paint is always the safest decision, using the testing guidelines suggested above.
You can get more variety and personality with borders and stenciling or faux finishes, without committing to wallpaper. Don't forget to consider wall lighting as well, and how the light will create its own detailing on the wall. Consider architectural detailing for ceilings, walls, windows, and doors at the same time. The style of moulding choices around windows and doors should blend together, but it's not important to match everything perfectly. Some detailing in the room can be stained, while other elements can be painted in a coordinating tone or classic white finish.