Ice sculpturing is a pretty demanding task, but at an amateur level it can be a pretty fun thing to do at a party.
A few blocks of ice
A few chisels and hammers
A few saws
Remember, this is supposed to be a fun contest held at a party, so don't go making it too professional.
Keep the ice chilled so it doesn't melt and let the party go on. At the beginning announce that you'll be holding an ice sculpture contest and let people sign up. When the time is right, give the people that signed up their tools and blocks of ice and let the contest begin!
You should set a time period in which the sculptures have to be finished. After that put up a ballot box and let the rest of your guests vote on which sculpture they like the best.