Here's a really adorable idea for making hot air balloons. Take some cardboard paper and cut out shapes similar to the ones in this photo. Next, repeat the process and fold your shapes in half before gluing them all together to make your balloon. You can use some square cardboard as a basket and attach them to your balloons using string. Finally, hang everything up, and you're all done.
These look really good inside, but we especially love the idea of hanging them from a tree, which is what they've done in this photo. Just make sure to coordinate your colors.