I shouldn't have written so quickly about not playing Halo 3 single player again: I forgot about the skulls. :)
If you're not familiar, basically Bungie has turned a few Halo 2 easter eggs into a full-blown feature of Halo 3: Now there are 13 of them, and each time you find one, you can alter the game experience in various ways, usually to make it harder. There are silver skulls, which aren't actually worth any achievement points, and gold skulls, which are worth 10 points each. Some are very easy to find, some are quite difficult and some even disappear when certain conditions are met.
Anyway, I guess I'll tool around with the skulls for a few days before completely giving up on single player.