Gather the artists at an outdoors table (or garage) and set up three pumpkins. Allow them 30 minutes (or so) to carve the best pumpkin, and let the guests decide on the winner. Have pumpkin related prize for the winner such as a pumpkin cookbook, or pumpkin scented candle.
Use only the "kid-friendly" carving kits, since drinking and sharp tools don't mix. Schedule this event earlier in the party and use the pumpkins to decorate the buffet table. And make sure participants don't feel too rushed or they may suffer a bloody injury.