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My favorite form of circuit bending, because it’s the easiest, is what I call circuit starving: stick a potentiometer into the battery connection so you can turn down the voltage until the circuit has just barely enough power to work. That’s where interesting stuff starts to happen.

You don’t need to open the toy’s case, solder, or cut anything for circuit starving. Make a sandwich with a piece of paper between two scraps of aluminum foil attached to the leads of the potentiometer. Insert it between one of the batteries and its spring contact, and the potentiometer becomes part of the power circuit. Then you can tweak the pot, in very tiny increments, until the weirdness happens.