Parallax makes a lot of interesting devices to interface to their Basic STAMP. Some of them are way overpriced, but here are some winners:
Piezo Film touch switch / motion sensor $1.49
Hall Effect sensor $4.25
PING Ultrasonic distance sensor $24.95. Three pins, VDD, GND, and and I/O pin for input to start the ultrasound pulse, and then output when it hears the echo.
PIR Sensor $7.95. Three pins (VDD,GND, and output), plus it is small (1" on a side).