There's one particular place where the stairs create a little slant in the wall. Alex thought it would be cool to put something there, but everything we had, including a few canvas paintings, were too heavy. Fortunately, fabric saved the day.
We dug through my fabric stash and found a piece of fabric we both loved. (It's an Anna Maria Horner fabric from her Innocent Crush collection.) I sewed a little 1/2" hem around the entire piece, and then we just tacked it to the wall. The whole thing took less than 15 minutes!
How do you hide ugly walls?