I've not been able to find a convenient online image showing the characters you get from holding down alt while typing, it may vary by layout, but for me this lets me type:
But of those, I only remember €, # (both printed on the key!), ∞, ƒ, ™, π/∏ (thanks to growing up with MacOS classic — Marathon Infinity for ∞, ƒ for folders, ™/π/∏ no idea why), and –/— (en-dash/m-dash, not sure why I learned them, but was one surprise source of compile-time errors around 2010 because they look exactly like - in a fixed-width font).
If you ever used MPW shell, a lot of those characters were part of the syntax of commands and the regular expression parser so it was common to learn to compose ∫,® ∂ etc. The debugger TMON also used them, so they just become second nature, like !@#.
Neat, did not know that. At the time MPW shell was used, it was a little bit too advanced for me — I was only as far as working my way through C For Dummies (or something like that) with a limited student edition of CodeWarrior* around the time REALbasic came out.
* Possibly bronze edition? Whatever it was, it was 68k only.
