Yep, you can't quite make it to those places, but it is fun trying!
There are quite a few unexpected places that you can go but I don't think anyone has bothered to put together a list. I've been on top of the Serpent Temple and all over all the walls at Shurugeon Castle. I tried to get on top of Skull Castle but don't think I ever managed that one.
Maybe we should start a list/thread.
Wyv