Most relaxing/absorbing would be the chisels and my hand-crafted wood rasp, while carving on a classical guitar.
Most nerve-wracking would be using a router to cut shelves on the guitar body for the ebony edge binding inlay: the surface is curved AND domed, and if my cut is off by as little at 0.003" or so then it seems to show as a pretty nice gap. Seems easier on this 4th guitar; was mostly white-knuckled on the first two!
Cheers,
Scott