Two-source ripple tank
Drag a probe around the tank and read how far the wave from each source has travelled to reach it.
Waves
4Young's double slit
Move a marker along the screen and read how far the wave from each slit has travelled to reach it — with light, with microwaves, or with sound.
Standing waves
Pick a harmonic on a string or in a pipe and see where the nodes fall, then drive it from a signal generator, sweep for the frequencies where it resonates, and listen.
Doppler effect
Set a source of sound moving, or the listener, and read the wavelength and frequency each listener receives; then see the same shift in the spectrum of a galaxy.