I've been experimenting more with pen-and-ink drawing as it's more portable than, say, digital art (I have Photoshop CS6 but only for my Windows desktop). I can take a sketchbook pretty much anywhere, and since I carry fountain pens anyway I might as well put them to use. For going out and about, my weapon of choice is a Pelikan M205 Aquamarine with a fine nib--it has great ink flow for keeping up with quick sketching. It doesn't do line variation to speak of, but that's okay. (I do have vintage pens with flex nibs, but I don't always want to break them out.)
This one was a line test to see how scumbling looked on an ashhawk/phoenix. I've been slowly exploring Alphonso Dunn's Pen & Ink Drawing: A Simple Guide and just the first twenty pages have already been worth the price of admission. Admittedly, I'm no artist, but art makes me happy and it's worth it for that reason alone. I actually really got into drawing because it was the only way I could "see" my story characters!