Games and Tools
Make any frequency or amplitude traveling wave on a string, and even play around with damping. Better than jumping rope, but you might burn fewer calories.
But educate yourselves on how waves interfere with each other—light, sound, and ripples.
How microwaves heat food and drink.
Videos
Plane traveling at supersonic speeds will often cause moisture to condense around them in wedge-shaped contrails that precede sonic booms, leaving the traditional Doppler Effect far behind.
Some of the people most in tune with wave interference are concert hall architects—who must make sure the performers sound phenomenal from every single seat.
Most waves we've talked about have small amplitudes, but they do get bigger. Much bigger.
Websites
If you've ever wondered where Superman's heat vision falls on the spectrum of electromagnetic waves, well, Randall Munroe's got you covered.
Another wave propagation comparison from xkcd—when does Twitter let you know about an earthquake before the earthquake itself?
How seismic waves fit into our transverse/longitudinal discussion.
This unmanned robot uses two very different kinds of waves for power—sunlight and ocean swells.