A simple balloon driven hovercraft, with large lip on the balloon tube to hold the balloon more securely.
 

#Edit#

I've started tweaking this in various ways.
Version 2 now has separate directional fins that can be pressed in and aligned to experiment with directional control.

The pins and holes are designed to be the same size, however the pin on the fin has 24 sides, so it should press in okay.

hovercraft.stl is a single print that doesn't require the additional fins.
hovercraft v2.stl requires 2 x hovercraft fin v2.stl that press in on each side.