The most sensitive and best way to do this demo is to use the arm hanging from a thread assembly.  Position the grapes on the ends of the assembly and move them in or out until you have achieved a balance where the grapes hang level.  Use the large magnet to move the grapes around. 

Using the rotating support is much more difficult as the system has to be in perfect balance which may take quite some time in some instances.  Take two large grapes and mount them on the rotating support arm.  You can move the grapes in and out to balance the support.  When balanced you should be able to place the Neodymium magnet within a quarter of an inch to the grape and the grape should move away from the magnet.  This shows the diamagnetic properties of liquid water.

An alternative is to fill one of the test tubes with water and balance it on the rotating arm.  When the large magnet is brought near, the test tube should be repelled from the magnet.