Ooh, this is tough; I've received a couple one-a-day challenges that are on things that I really do need to do. Jimmy Norton invited me into the Flickr Project 365 challenge, which is to post an image a day for a year. I am always dismayed to realize how little I photograph Hawai'i, which is an incredibly beautiful place undergoing profound changes. Plus, if I did more photography, I'd have an excuse to buy a digital SLR.
Worse, Dan Ciruli called me out for his pushup challenge, which is "do 1 pushup and 1 crunch on day 1, 2 on day 2, 3 on day 3, etc." I hurt my back for the first time in my life a few months ago and know that the lesson is that I have to strengthen my core (#1 way to help your back: strengthen your gut). Plus, although I'm pretty fit aerobically, I have very little upper-body strength, so pushups are good for me, too.
I wonder if I should counter-challenge: "On day 1, swim a distance of 1' underwater..."
(Photo is me on the bottom at \~80')