We are definitely fan of the paper airplane here. We've done Strawplanes, oversized planes, and dragon planes. Well here's a new one to try - catapult plane from Minieco that they saw in The Usborne Big Book of Science Things to Make and Do. Add a rubberband (and some tape or cardboard for stability to the bottom of your plane) and see how it affects your launch!
Let me know how it goes if you try it out soon!