Have bowls of balloons. People pick the balloon that matches their guess for the gender. Poke holes in the balloon matching the color that is not the gender, and have everyone try to blow up the balloons at once. Only the correct balloons will blow up.
What a great idea! How the gender of our baby was revealed at a party with family around Easter! My mom and sister dyed two sets - one blue one pink - and filled the eggs. We took them with us to the ultrasound and the tech sent the proper bag home with us. NO ONE at the party knew the gender until we opened the eggs! It was soooo much fun!!!