Here is how I learned the function of a capacitor. (in the dark ages)
I think a great way to explain capacitor... and have it mean something... is to make an oscillator.
Lets say it's a multi-vibrator type oscillator.
- easy to build.
- 4 resistors, 2 capacitors and 2 NPN transistors...
- not hard to breadboard
- all basic cheap stuff.
You adjust the frequency of oscillation by changing the values of the capacitors and you can explain that each part "value" has a specific "capacity" for a charge and as that capacity changes... the frequency changes. Allowing the explanation of the common Resistor/Capacitor charging circuit and the fact that the transistor "fires" when the charge reaches a specific amount.
Much better, but more technical explanation in the wiki.