One cheap easy thing I did for the walls was to buy some canvass from lincraft then I bought pretty patterned material and stuck over the top. They just brighten the room and cost me less than $20 for 3.

As said e-bay is great.

And even a coat of paint in nice color can make huge difference, we renting so couldn';t do that but friends places I've seen just a bright color painted totally transfornmed room.