While it's not going to take the place of a large power sander, for those small sanding jobs you only really need a cheap motorised toothbrush and a few other items for a cheap DIY option.
DIYer Quinton, posting to his personal blog Garage Publications, came up with a neat idea. If you just remove the bristles on the head of a motorised tooth brush, apply double-sided tape to the head, and then top it off with some sand paper, you'll have your very own miniature power sander. With all the electric toothbrush options available, you can pick up a cheap one that runs on AA batteries for under $10, or opt for a nice model with rechargeable batteries.
DIY Electric Toothbrush Sander [Garage Publications]