Another really handy thing is the Compose key. If you're using GNOME it's under Keyboard Settings, under Shortcuts / Typing. I have it set to Right Alt. The idea is there's just a whole bunch of memorable key sequences for various common Unicode characters. For example, Alt + o + o = °; < + 3 = black heart, < + " = “, etc. It doesn't have all of the ones you like, but it's helpful :)
I usually map my insert key to compose -- I never use 'insert' for the default functionality, but I also don't type funny characters often enough to justify getting rid of an alt or control key.
I love reading about how different people map different things. For me, it would be a disaster to map insert since I use that for pasting (shift+insert--I'm a lefty and it's a bad habit I grew attached to years ago).
My choice for Compose is the right Windows key, which I think I eventually settled on because I use the left one in some keybinds (winkey+s for shell, etc.) and like you, I couldn't part with an alt or ctrl. I've often wondered what other folks tend to use.
To the grandparent: I'm sometimes amused by what Compose defines. There's ∞ (compose + 8 + 8), (compose + # + #), and oddly (compose + C + C + C + P). I think it may depend on system configuration, but I believe libX11 is responsible. (On my system, Arch, the key combinations appear to be documented under /usr/share/doc/libX11/i18n/compose for my locale.)
