I was trying to explain how a capacitor works to someone who considered themselves a "mechanical".  I used the hydraulic model of a hollow metal sphere with a rubber bladder separating it in half and a port connected to each half, both halves filled with water. 

Voltage is pressure
Current is water flow
What is capacitance? 

Maybe the model breaks down when it comes to capacitance.  The best I could come up with at the time was capacitance is the stiffness of the bladder.  The more elastic the bladder, the more flow produced to by a specific pressure.  But now I think there is a flaw here but I can't quite put my finger on it. 

What would charge be in the hydraulic model?  Volume of the water? Mass of the water?  Should you use a compressible fluid instead of water?  

Your hydraulic capacitor model is right on track, I've used the same analogy before. The larger the diameter of your sphere, the more capacitance. The elasticity would be more on the line of the voltage rating.
You can demonstrate DC and AC characteristics quite nicely. Draw a big open tank full of water (the ground) then draw a centrifugal pump with the suction in the tank. Draw a discharge pipe with a little turbine in it with the discharge going back into the tank. Draw a handle on the pump and show how as you crank the pump, the turbine will turn.
Now put your hydraulic capacitor in the circuit and show how when you first start cranking the pump, the turbine will turn momentarily but will stop when the bladder finally stretches to its limit.
Now replace the pump with a piston that you can move back and forth with a lever and put another piston where the turbine was and show that as you move the drive piston back and forth, the other piston will also move back and forth even though the water is only sloshing back and forth in the pipe.
You can demonstrate a diode with a check valve. For the pi