Snowball fights are a classic part of every winter... well... if you don't live in a tropical environment.
They can also be a fun thing to do at a Christmas party. You can just start a spontaneous snowball fight by throwing a snowball at someone, or you can take it to a whole new level by making an organized snowball fight.
Make two teams that will be fighting each other and divide the territory(for example, each team gets half of the yard).
Give the teams some time to prepare. During this time the teams can make some extra snowball so that they don't have to make them on the fly. They can also build walls for protection, maybe even a bunker. Basically they can do whatever they want.
Let the fight begin!
Announce the start of the fight and let the carnage begin! Let those snowballs fly with speed and accuricy until the whole backyard is dug up!... then go in house and discuse truce over a cup of hot chocolate:)