I found this idea on the Roberts Crafts blog, http://robertsmediateam.blogspot.com/feeds/posts/default, and thought it was super cute! It's a canopy from IKEA and decorated with fabric and mod podge.
Here's a link to the instructions:
I've heard that the canopies at IKEA are very inexpensive, but I haven't actually been there yet, so I don't know for sure. There are a lot of other great ideas on the blog, I just really liked this one.