I really like silhouettes for this type of thing. You can usually find interesting silhouettes online by searching or by blowing up a wingding font - right now I have a canvas hanging in my dining area that I painted and traced with the outline of a bird, then filled it in with more paint. It was super easy and no one has to know it was traced.
The other thing that I've done that looks pretty cool is to take some coordinating paint colors and paint little circles, eventually making enough little circles to make the shape of a big circle. Then add a skinny trunk and a line for the ground, and voila, a big fat colorful tree!
Also, I have seen people frame pieces of big, fancy wrapping paper on the wall. Y'know, the super nice stuff you can get at places like Barnes and Noble or Paper Source? This can look pretty good, too.
Another idea is framing pages from old books or dictionaries... I have done this with pages I like the typography on and it sounds a little hokey but doesn't look as weird as you'd think. I have also seen people find funky shaped frames at thrift stores, paint them in loud colors, and hang them in clusters with nothing in them. I think it looks pretty cool
Finally, I have some "swatch portraits" (like these: http://www.purlbee.com/swatch-portraits/
) hanging in my sewing area - that's an idea, too.
Hope that helps!!