There are a lot of places in World of Warcraft. Sometimes, I log on late and just go around exploring them. I like to fly around Ahn'Qiraj, or check out the area around Stratholm (sometimes I kill stuff, sometimes I just look around) -- I once spent an entire night just hanging out in Uldum looking at the cool architecture and flying around exploring. Heck, I still miss wall-walking because my wife once organized an expedition for our entire guild back in the pre-BC days, taking us all around the Wetlands and eventually showing us how to get to the road to nowhere high above Menethil Harbor.
One of the reasons I'm looking forward to Warlords is that, so far the two zones I've seen have been crazy fun to explore (if you watch out for the level 100 ogre elites near the top of Frostfire, anyway) and I'm looking forward to exploration making a bit of a comeback. So I ask you - what's your favorite hidden corner in WoW? Or do you ever just log and go for a ride?