Son's science project (high powered fans)
Getting a "shelf" (whatever that really means) to fully levitate will be quite hard.  There are issues that are out of reach of a 5th grader, and also you, judging from your question.  Not only is quite high thrust, and therefore power, required, but keeping the shelf stabilized and not flip over and drive itself into the ground is a advanced control system problem.

However, a hovercraft is much easier.  You can make simple ones with simple materials.  A stryrofoam cafeteria tray upside down is a good start.  You can use a single propeller blowing backwards to propel the craft forwards.  Divert about half the flow from the propeller to under the tray to keep it floating.  I've seen this done by gluing half a Styrofoam cup over a opening in the tray.  The half cup is oriented horizontally, with the open end right behind the propeller.