Although I don't do this every day (although I should), I try to go bicycling at least a few times during the week. It's a good way to clear my head. No calls to take, or meetings to think about. No paper to jot down ideas. They're all in my head.
I'm lucky in that where I live I can go biking along the sound and well, there's something about being by water that just has a naturally calming affect on me.
I'll mull over things I need to do and maybe think about a particular problem I need to solve. Funny thing is that even though I'm not able to write it down as I think about possible solutions, because I've been thinking about it, I can still remember it after riding and quickly write it down.
Then again, even if you use the bike ride to just get out in the fresh air to enjoy a portion of the day, that's all that really counts. If you can be productive at the same time, bonus!