One of my favourite instance entrances in-game has to be the entrance to Onyxia's Lair.
Day 27: 10 minutes from home
For this one I was inspired by an article on WoW Insider a while back Have you ever gone for a walk?. I started off directly below Dalaran city and followed the road around to the south and into Dragonblight. I walked for 10 minutes and this shot is taken from where I finished - in the Crystal Vice.