Most portable electronics (GPS, cell phone, battery chargers, etc) can be powered by automotive 12 volt adapters. Pair a 12 v camera battery charger with a solar 12 v trickle charger. Hard wire the leads to each other to eliminate the weight and bulk of the cigarette lighter plug/socket.
The trickle charger (typical 1.5 amp output in good sun light won't supply a quick charge but given that you'll be outdoors much of the time a solar collector attached to the top of a pack charging spare batteries can be a big help. And it can service a number of different devices with suitable mobile chargers. (Pairing items that all use AA/AAA batteries is efficient and is why the K200D remains a primary travel kit choice for me.)
Harbor Freight has an inexpensive solar unit which can be removed from the bulky plastic frame if used carefully and there are more expensive flexible panels that might serve even better. Matching the 12v solar output to your mobile device charger(s) is the solution.
One example:
1.5 Watt Solar Battery Charger
H2