A few years ago at my old job we had game fridays once a month. The whole office would get together and play a day long game. We did a Murder Mystery one time in which each office worker was killed and you had to guess who was the axe murderer. One of my favorite games was creating a Mini Golf Course. Each person either in a cubical or an office had to create his/her own Golf Hole. Some people went all out with mad ass creative designs, some went for making it almost impossible to get a hole in one and others just placed a cup on the ground and called it a day. Needless to say golf putting is fun!
So anyways...it seems YouTube has an online competition called "The Best of US Challenge" LINK. It's basically a serious of videos of famous people doing some challenge that is unrelated to what they are famous for and then calling for viewers to make their own videos beating that celebrity.
As for Phelps here is the gist of his "The Best of US Challenge":
Speed putting, however, is another matter. Not that I, personally, would be any better at it, but perhaps some of you could beat Phelps in that particular discipline. And if you do, you could win a trip for two to the Vancouver Winter Olympic Games in February 2010 and a trip for two to the Youth Olympic Games in Singapore in August 2010. - source
Below is the Michael Phleps video. I may have to try my luck at this.
