Are you the guy who is using the public loo? Or the one telling everyone he's on the john?

Keep it simple. You could talk about standing waves on a string. If you take a jump rope and shake one end, the wave travels down and reflects off the other end (if it's tied). When the original wave and the reflected wave meet, they interfere.

When you hit the right frequency, you see the points that don't move at all (called nodes), and the points that move the most (antinodes). The nodes are locations of perfect destructive interference because the wave going one way and the wave coming back are always canceling out at that point. The antinodes are perfect constructive interference because the crests and troughs are always adding up there to give maximum displacement. It's a great example because you can actually see the result of the interference pattern without needing special equipment.

Definitely use thin film interference. It’s super visual and technically correct for light waves. You can just look at the colors on a CD or DVD too. The grooves on the surface act like a diffraction grating, but it’s still fundamentally about light waves hitting surfaces and interfering.

For a destructive example that everyone knows, you could mention the dead spots sometimes found in indoor arenas or theaters. Those are specific frequencies getting canceled out by reflections off the walls, floor, and ceiling interfering destructively with the direct sound. Acoustic engineers spend a ton of time trying to fix that kind of unintentional destructive interference. It's an annoyance caused by physics, not an intentional cancellation.

You nailed one of the best examples with noise cancellation, that's pure destructive interference in the sound domain.

For constructive, a less common but really important example is in antennas and radio signals. Think about a directional antenna array, like the huge ones for AM radio stations. They often use multiple towers (dipole antennas) spaced very precisely. The signals transmitted from these towers are timed so that in the desired direction, the waves all arrive "in phase" at the receiver. This means their amplitudes add up, creating a much stronger signal pointing that way (constructive). At the same time, they are often designed so the waves cancel out in other directions (destructive) to avoid interfering with other stations or wasting power. It’s a very precise, real-world application of wave phase manipulation.

Don't overthink it. Think about your sound system. If you have two speakers playing the same song, there are spots in the room where the bass sounds really strong and other spots where it seems to disappear. That's interference.

The strong spots are constructive interference because the sound waves (especially the long bass waves) from both speakers are hitting you "in sync." The weak spots are destructive interference. If you walk around the room while a steady, low tone is playing, you can literally map out the areas of high and low sound intensity. It's just waves adding and subtracting based on your distance from the two sources.