Since level 40-50 instances are boring (god, zul'farrak, fuck that shit) I've been spending most of my time lately SPLORIN'

Under Stormwind! There's a "seam" between the wall and the hill near the gryphons where if you jump just right (i.e. are willing to spend half an hour or more swearing at your computer until you get the feel for it) you can get underneath Stormwind. There's really nothing to do down there, it's just massive blank space up until you get to the edge of Dun Morogh, where there's a big white wall. There's also a hole in the ground, and if you jump in, you either DIE IN BLANK SPACE, or land in the Wizard's Sanctum (that place you teleport to in the mage district!).