Maybe a cell phone. I don't carry one, but my commute is only 5 miles. No matter how well prepared you are, you might be stranded along the route sometime. If you have someone to call for a rescue, a cell phone would be handy.
I'd look for a cheap water-proof bag. I have a couple vinyl coated nylon ones. Until you find a deal on those, use large freezer grade ziplock bags for stuff like clothes, food, and cell phones.