electric chewing gum prank circuit diagram
Here's a little circuit that might fit the bill

The battery charges the capacitor (C1) through a resistor (R1). When the switch is operated the charge from the capacitor is dumped into the primary of the transformer. This produces a damped oscillation (LC circuit). The transformer secondary has a lot more turns than the primary and so the VOLTAGE part of the short burst of alternating current in the primary will be magnified by the turns ratio. The current part will be diminished. The total energy delivered to the 'shock' can be estimated from the energy store in the capacitor. (0.5 * C *V^2). For the values given = 0.5 * 22 * 10^-6 * 9^2  Joules or 891 micro joules - pretty harmless. The transformer is a small audio output type but others can be used such as the trigger transformer for a flash tube (hacked from a cheap camera) or you could even wind you own on a ferrite ring.