In vanilla, I spent one full night just swimming around the Eastern Kingdoms on my druid. I'd recently heard rumors about the Ashbringer possibly existing in the game, just waiting for some wandering soul to come across its incredibly well hidden home, possibly requiring high level fishing skill to attain. Despite the fact that the scenery was mostly repetitive and to most anyone else should have been incredibly boring, I really enjoyed the evening.

I later became a Game Master and had the ability to fly invisibly through the terrain on live servers and occasionally would make a point to travel to random nooks, either not normally accessible or else very hard and time consuming to reach by legal means, and that was fun too. This was before flying mounts: characters would appear to swim through the air and could be made to travel very fast indeed.

The best hidden secrets in the game were things like The Chair, at the base of The Room, an underground dungeon that could only be reached by flying through the terrain of a castle that stood on Game Master Island, which itself could only be accessed using Game Master powers. Rumors were that early rule breakers in the game, perhaps in beta, were transported into The Room where all they could do is sit in The Chair or not. I don't know if that's true.

Probably people discovered all this on private servers as well, but there was definitely something magical about doing it on live realms - at least for me.