Yesterday I organized our first CodingDojo here at the company and I'm glad to say it was a great success!
The problem chosen was the Karate Chop, from Dave Thomas' Code Kata.
As it was also my first time organizing a Dojo, we took some time to get things straight, explain everyone the rules and start coding. Nothing that would compromise the experience, though.
The rules were basically the ones of a RandoriKata and as a mechanism for switching pairs we chose the TimeBox with a limit of 5 minutes.