Technical pen, digital colour
I've taken to making designs in the style of medieval Celtic knotwork as a form of mental relaxation. The process is quite meditative, requiring just enough brainwork to keep me engaged, while being mechanical enough to allow the mind to wander at will. This sort of thing is reasonably simple to throw together; the next step is to investigate how to build the zoomorphic and anthromorphic figures used in Celtic illumination. They're trickier, and consequently more rewarding.
I doubt that I'll ever get heavily into this sort of thing though. It's decorative enough, and quite fun to do, but it's a style of art whose time has passed; those obsessive 6th century monks have all pretty well exhausted the possibilities of the form.