Physical effort in outputting 5V 1–2A?
20 watts delivered into a hand-cranked generator (enough to deliver 10 electrical watts after conversion losses) could be managed all day by a fit and motivated person. This would employ all of your arm muscles. However, you'd need reasonable length cranks, comfortable handles, a good secure mounting for the generator, it's not something you'd nonchalantly carry onto and use on a bus.

A grip-squeeze device could be rigged to generate some power but, only being able to use the small grip muscles in the forearm, you'd struggle to get more than a few watts for a few minutes before exhaustion.

If you don't mind standing, then a mini-stepper could be rigged with a generator. This would use your legs, so we're back in the sustained many 10s of watts, even 100+ region. It could lie stably on the floor, and maybe fit into a small backpack, so is probably the most suitable device overall. Consider that a 70kg individual walking up 200mm high steps one per second is delivering 140 watts.