One of my "go-to" choices are drapery panels. They're fairly inexpensive and come in various lengths, style, and color choices. They can soften the look of a room while elongating the window and adding so much dimension.
Another popular choice, is a roman shade. This versatile shade is fabric and has wooden slats sewn within. So when the shade is pulled up its like a valance, and when pulled down it provides good privacy like a tradtional shade. This shade adds a little extra to your window, but not quite as dramatic as drapery panels. If you're crafty there are several on-line resources which can instruct you on how to create your own custom shade. For the advanced DIYer try your hand at making a cornice board.
As with any design choice no lives will be lost if you choose the wrong window treatment. If you are still not sure what to choose, purchase a few options (different styles, colors, and materials), and see what works. You might be pleasantly suprised at the impact...so stand back and be amazed!