ROUGH NOTES!
CIRC KATA TU, and other fun
Superscript numbers
Thorn, maybe eth
Various arrows, especially right arrow
Fractions, copyright symbol, trademark symbols
Keyboard symbols
Interrobang
Greek letters, especially pi and mu, maybe a couple of others
Musical notes
Tick mark and cross?
Maths symbols, times and divide especially
Dashes
Utility polygons, circles, triangles, squares
Some Apple colour emoji
Heart, smiley face, pilcrow, star, big quote marks
Various combining diacritics
Issue: Arrange a cheat sheet logically by kinds of character, or by input method, or somebody meld the two?
On the standard UK Macintosh keyboard layout, there are:
The obvious way to get more characters out is by increasing the number of modifier keys and what they do. I've often thought that it would be nice to have different command key for shortcuts taken by the OS than for those taken by applications, for instance.
The option key on the Mac keyboard is better than anything on Windows, but still limited. For many things, something like a compose-key would be better, so if you hold down Compose then hit o then c you get ©, for instance; or e then ` becomes è. Once you've done that there is all sorts of fun stuff you can do.
An interesting idea in Mac OS X Lion (originated on the iPhone) is that if you hold down a key it will pop up box showing variants of that character.
Perhaps use this instead of introducing modifier keys! It would be a little slow for long typing, though. The Space Cadet keyboard could at least have the Greek key held down with a paperweight or something if you were doing extended typing in Greek, whereas this would require you to hold down each letter until the box popped up end then hit the number of the character you want.
Keys should be mnemonic and have simple relations. Alt+S could be long-s, and Alt+Shift+S could be eszett. Then again, you do get conflicts: Alt+T should obviously be thorn, and Shift+Alt+T should therefore be eth. But what about capital thorn, capital eth, theta, capital theta, tau, and capital tau? We need more modifier keys!