One of the quickest, simplest and most affordable ways to change the whole look and feel of a room is to change the window treatments. Wallpaper, fresh paint, or new furniture will, of course, nicely do that too. But changing the window treatments is something you can do yourself in no time at all, and the visual rewards are immediate.
If you prefer to have the focus of your room be away from the windows, and toward, for example, your furniture, or a colorful rug or wall hanging, the obvious choice for the window is a natural or neutral color, and a relatively simple style. Even a simple style, however, can be graceful and pleasing. Plain curtains can be tied back in the middle, down low or up high, depending what looks best to you.
Some people prefer more of the room focus to be on the windows, to complement or contrast with neutral color in their furnishings, or to emphasize a view out the window. In this case, perhaps more color or a bolder style in the window treatments is called for. Layering a top treatment over a curtain or a sheer, for example, definitely makes more of a statement at the window.