The simplest way to reduce the voltage is to put a "load" resistor across the piezo terminals. The current-limits of the device will reduce the voltage. You'll have to experiment, but if you don't know Ohm's Law, look it up... It defines the (simple) relationship between voltage, current, and resistance.
In addition to a load resistor, it would be a good idea to add a "protection diode" to the input, so you don't burn-out the electronics if the voltage happens to go higher than expected. The diode goes between the electronics input (Arduino or op-amp, etc) and the power supply (probably 5V), and it's wired so that current flows into the power supply if the input voltage goes higher than the supply (plus the small voltage drop across the diode). Under normal conditions (with the input lower than the power supply) no current flows backwards through the diode.
The op-amp circuit that holds the peak input-voltage is called a peak detector. It's just a an op-amp, diode, and capacitor. An optional resistor across the capacitor will slowly discharge the "hold" voltage.