Each team needs to be given several balloons of different colours, shapes and sizes. Ask them to work out which balloons will travel farthest just by looking at them. Now inflate each balloon and let it go.
Which one went farthest? Why is this? The balloons will go in lots of different directions and the ones you expect to go a long way may well not.
This should lead to a discussion about how to make the test more accurate.
This may yield a number of suggestions, but here is one that works well.
Thread a small piece of straw onto a length of thin string that will stretch across the classroom - making sure the straw can slide easily along the string. Fix the string from one side of the room to the other and pull it taut. Take the balloon you wish to test and inflate it, pinching the tie-end shut. Measure the balloon's dimensions. Attach it to the straw with sticky tape, making sure the end the air will escape from is pointing backwards. Let go. Measure how far the balloon travels.
Now try each of the balloons in turn. What affects how far they go? If the balloons all travel to the far end of the string, attach a load to the straw and try again.
Another great suggestion is to use a balloon helicopter, a cheap plastic toy. Once you have attached the balloons you can measure how long the helicopter stays in the air.