Here’s a YouTube “howcast” video which teaches you how to make beatbox sounds;
Many of the Olympic Sports could be used as a starting point for beatboxing.
Football Race (emphasise the “ss” – it will naturally syncopate) Discus Archery Pentathlon Shoot(ing)
For a full list of sports, try the official Olympic site at www.london2012.com/
Once you have chosen your sports, create your beat box sounds by arranging them in an order that makes a good rhythm. All you have to do now is manage to say them….