There was a recent thread on gadget power options that examined the dynohub and solar power options and I got the general impression that the dynohub approach is heavy and still a bit hit and miss and the efficiency of solar cells isn't quite enough to make it a satisfying solution.
So IMHO the obvious solution is to simply take a spare external battery. There are now 10000mAh batteries that will charge an iPhone 8 times. This should be enough power for most tourists' applications until the next wall socket is available.
There are 3 options that I've found and I wondered if anyone has experience of these or other recommendations