A word of warning, not all phones will charge directly from a dyno connected USB. Starting and stopping can also lower the efficiency as the phone wakes up each time. Tablets are more picky. Battery lights usually will not turn on while charging. A USB battery will often charge off of anything. I don't know if bicycle GPS will charge and run at the same time, but I recall reading one complaint.

Solar sounds like a good option for this application, maybe on top of the handlebar bag. A dyno is wonderful for frequent and always ready applications like commuting and randonneuring, but pretty pricy. Other dyno's are easier retrofit, but usually loose out in efficiency, though still not so noticeable with today's low power needs.