My favourite thing to do in Halo 2, as I've mentioned in a few threads about it before, was sword cancelling with friends to get to areas of the map that you weren't supposed to get to.
Honestly, I had much more fun doing that than actually playing the multi-player. Getting up rockfaces; climbing up buildings; having someone crash a banshee into someone with their back against the wall so they went flying up the map. So much fun. Only other thing I did in a Halo game that was remotely as fun was Speed Halo in Halo Reach:







