Sensors that are also actuators?
Examples:


(As you said) a piezoelectric transducer. It can produce sounds, or work as a microphone.
An electromechanical speaker. The same device can act as a speaker, or as a microphone (old walkie-talkie toys use that fact).
Even an LED. If you apply forward current through an LED, it produces light, and it is an actuator. If you bias it with a reverse voltage, and measure the reverse current (very low), you can measure light with it, and it will be a sensor.
An antenna. It can transmit and receive (even simultaneously, at different frequencies, or using other discrimination schemes).
An electric motor. You can use it to produce and to detect (and measure) movement.
Even an NTC/PTC resistor (which is supposed to be "only" a sensor), could also work as an actuator, if I inject enough current so as to modify its temperature, and that of its surroundings. I could use it to keep a (very) small mass at a constant temperature, in a low ambient temperature environment, and a single device would suffice as a sensor and actuator.