well there's 4 that come to my mind... and doing variations among them should be good for keeping your general shape.
jelqing (manual friction/stretching exercises, meant to be done when at varying stages of hardness, lots of reps needed, and hours at a time, but it's free)
pumping (meant for girth mostly, until you're at the sides of the tube already in which case there's only one direction to go and that's to the end of the tube less time needed (15 min/day, every other day advised for healing))
stretching (device based, take a long while, meant for length, can be done while asleep as long as positioning is comfortable)
hanging (weight attached at the end, with a device clamped on to secure the weight, mostly for length in the shaft)
upon further inspection I see you've got pumping mentioned already... from what I hear, keep the Hg at 5 and under during the session and you should be fine.