Uh, everything in Northrend?

No, really. Yes, I'm serious. I did them all (just like I did solo on my other guys) and ended up having enough gold for Swift Flight Form about 1/4 through Icecrown after hitting 80 in Storm Peaks (which left the last half of Storm Peaks and all of Sholazar/Icecrown for $$$). Once I wrapped up the last of the 5-man stuff in Icecrown, it took about 2 days of dailies to get enough to buy the extra mats for epic pants/boots and 5x Titansteel Guardians and a few other shinies here and there. I'm very happy with it, and it didn't take me forever to hit 80.

[edit] As for quests that are great fun to multibox, the 5-man stuff in Icecrown was hella fun after having so much trouble trying to 2-man it with a friend of mine. :P