I guess it depends on the type of productivity you're after. You could conceptualize software or schedule design through visualization, for example. This has worked really well for me. Or you could pick a learning topic which is amenable to travel, like learning to play poker on your phone or practicing morse code by tapping out signs and license plates onto your legs.

Really though, the details are going to matter here. Usually if you can get more detailed about exactly which details don't work, you'll find better traction faster. Good luck.