Balloon Toss
Form teams of two players and have them stand facing each other about arm's-length apart. Hand one player an inflated balloon and have him tap it to his partner. After one tap back and forth, players must take a step backward to make the distance greater. If players let the balloon hit the ground, they're out.
This also can be played with water balloons being tossed back and forth. If a balloon hits the ground or bursts during a toss, the team is out.
Room Volleyball
Divide the room in half with something as simple as a line of tape across the ground or a string as high as you like. Divide players into two teams, one on each side of the net, and have them play volleyball with an inflated balloon. Set a rule allowing only a certain number of hits per side before the balloon is hit to the other team. The team that lets the balloon hit the ground loses a point.
To make it more fun, make hitting the balloon with your hands illegal. Players have to get the balloon back over the net using only their heads, feet or other part of their bodies.
Stomp Attack
Blow up a pair of balloons for each guest and tie strings to the balloons' ends. Tie the strings around the guest's ankles, allowing them to drag a little behind the guests' feet. The start the stomp attack. The goal is to stomp and pop the other people's balloons while keeping your own balloons intact. Once a guest has lost both his balloons, he's out. The winner is the last person to have at least one balloon intact.
Balloon Stuffing
Form teams and have one member of each team put on extra large, extra baggy pants and shirt. Other team must stuff as many inflated balloons into the clothing as they can. Start a timer and let the teams begin. Be sure to have enough balloons blown up and that they are all approximately the same size. The winning team is the one with the most balloons stuffed into a teammate's clothing.
Balloon Sculptures
Make teams and have them try to build the best balloon sculpture by sticking balloons together and forming shapes. Give them all some double-sided tape.
It's best to let guests blow up their own balloons so they can vary sizes. You might also buy some balloons in different shapes.
As a variation, see which team can make the best balloon costume--make sure one of the players puts it on.