How about a hot air balloon contest?

Have students construct hot air balloons and create a contest. Some ideas for judging include: predicting the 'hover temperature', the ratio of the balloon's mass to the largest mass it could lift, aesthetics or school spirit, and application of chemistry concepts in explaining how the balloon works. Let other classes or the student body judge the contest. Learn more about how hot air balloons work at http://www.howstuffworks.com.