I did a fair bit of
research into this, but it was for a different application. The principles are the same, though.
Solar power is going to be limited, depending on your particular situation. Based on my research, most of the inexpensive solar units are going to take quite a bit of time to charge a battery...it's not going to be your typical 3-4 hour charge, especially if you're doing more than one. Plus, you have to erect a solar array in the middle of a music festival...good way to get ripped off, IMO.
I did some testing with an external charging battery and it's the way to go for a shorter trip like that. I was able to get 8+ full charges on Kanger VV 1100mAh batteries from an
Anker 15kmAh E5 battery. It small, fairly lightweight and discreet...I'd probably suggest two for redundancy and length of charge. You get a fair bit of loss in the transfer, 50-65% efficiency is about what you can expect on a decent quality unit. Just another FYI, it's probably not practical to recharge these batteries in the field...mine takes over 24 hours to regain all it's power. Here's my test results:
Vehicle charging may or may not be an option for you, but if you're invested into 18xxx series batteries, NiteCore makes a
4-port charging bank with a common 12VDC automotive plug. Just remember, it can run your vehicle's battery down to charge many hours in a row, so it may be necessary to run your vehicle for some of the time.
Good luck!