So, you start by getting some shotguns. Big ones, like 12 gauge. Then you get some clay pigeons. Then throw the clay pigeons and people try and shoot them out of the sky with the big shotguns.

What could possibly go wrong?

On second thought, how about a Super Hero Party?
People can come as their favorite super hero, existing or made up. If they are triathletes they probably have plenty of spandex to pull this off pretty easily.