Okay, so I've fully explored 2 of the 3 secret dungeons (Graveyard and Mountain Wilderness). Looks like I've gotten all the runes and staff for the dungeon in the Northern Wilderness (clicking on the rock gives 'no effect' now) - but where in the heck is the Mushroom Ring telport?

Side note: what's the max level requirement for a bard instrument? I've got a Mook Bard at lvl 22 right now, and I'm thinking about swapping to a fighter-type (For the Giant Sword of course).

Hi Caliburn!

If you touched all the runes and sticked the staff into the stone, then the mushroom patch should have appeared quite close to the stone, in fact, I think you can directly see it from there. Walk around the ruins in circles (well, just where the trees are), you should definitely find it there...
For the bard: Quite late to change, I changed my at level 19, but it was a good decision, and, well, for the Giant Sword... go ahead, bard instruments have a maximum requirement of level 18, and after that your bard won't develop much, so it is good to let him have a second career. Especially with such a nice tool ;)

Never forget your towel, cheers
Philipp