I like all the stuff you talked about. An option you might consider in addition to or instead of gripper work is the standing barbell crush. You stand with your hands hanging in front of you holding a barbell and crush it by opening and closing your hand. Don't let the barbell roll all the way down your fingers just part way as you squeeze your hands as tight as possible. You can work up to some pretty solid weight on this. I have been working anywhere from 5 reps to 20 reps at a time. The exercise works similar muscles as grippers but allows you to train both hands at the same time and saves on the tendons.
I still do gripper work in the car and I also do extensor work in the car with a heavy rubber band. Like you said little energy expelled and very portable.
What we think, or what we know, or what we believe, is in the end, of little consequence. The only thing of consequence is what we do. -John Ruskin