The web, as originally envisioned, is a unifying platform that works everywhere on any device. Have you come across some tools that only need a web browser to run in (no special software or plugins or specific browsers) that made you say, “Wow, I never thought you could do that in a web browser?”
These examples are just as some starting examples, ones I have used before in workshops.
Inspirograph- a web version of the old Spirograph http://nathanfriend.io/inspirograph/
Also - for interactive web art http://weavesilk.com/
Try web-based musical instruments, like WebSID https://www.igorski.nl/experiment/websid/
Or visual storytelling like Short Trip https://alexanderperrin.com.au/paper/shorttrip/
And one of my favorite ways to show that Wikipedia is a living, growing entity, the audio visualization of current activity via Listen to Wikipedia http://listen.hatnote.com/
What have you come across in your web browsing that makes you stop and say “WOW”?