I experimented with file storage boxes on top of my work desk (which is affixed to the cubicle walls, so can't be raised on its own) before committing to a standing desk. It looked like hell, and I got some funny comments from people, but I got to try it out first, and I loved it. So I started looking for better materials to put on top of the desk, and ended up finding a coffee table at a thrift store that was the perfect height and length. Paid $8.00 for the thing, and I couldn't be happier.
Oh, and I did invest in an adjustable-height stool from Target, one of those hard plastic space-age looking things. That way I can sit for a while when I want to, but it's not so comfy that I melt into it.
I feel it's made a huge difference in the way I feel at the end of the day. I still don't like being a cubicle rat, but I'm not absolutely comatose anymore.