LEDs my friend, LEDs. Some of these would take up about as much room as a fountain pen would in your bag. Plenty of USB-powered lights out there now too.

One of my Fenix CL2 neutral white LED camp lanterns often finds its way into my suitcase. It's barely 3"x2", puts out a pleasant light (not harsh blue/white like most LEDs) and runs a long time. I've found all sorts of uses for it on the road.