Yeah, that's a way too wide question. My first thought after reading news all day was: is he going to join ISIS? laugh Seriously, typical volunteers are usually invited by those countries in need somehow and definitely not for just surviving off the land somewhere in a desolated desert with no people around. Almost certainly some survivable living conditions will be provided for them. So I would concentrate on:

1. Personal entertainment. For me that would be a decent tablet with plethora of software, games, ebooks, manuals, music, video, learning courses, podcasts collection, family photographs... But usually Internet is also available at least at some spots around, even deep in the Africa. But, perhaps, you might consider investing in a long range directed WiFi link extender. Make sure you know how to install it, and take it with you (there are models you don't have to hook to the base camp router, just arrange its powering with the owner of the base hotspot). Don't forget about the solar charger (7-40W) and a battery powerbank. Though, most likely you will have a normal power outlet in your shelter, - the power may not be readily available when you need it (e.g. in N.Korea the light is centrally turned off at night for energy conservation).

2. Personal health. I'd take all possible immunizations viable for the region beforehand. The SteriPen for water sterilization (entertainment charging solutions above will make it work indefinitely, just make sure you have at least 2 spare sets of rechargeable batteries for it) - mandatory in equatorial countries like India. Assemble a serious enough emergency FAK with trauma kit and chronic meds you need. Stock on multivitamins for the period. Get most durable outer clothes and footwear you can afford for the expected climate.

3. Making friends. Make your "entertainment kit" suitable for enjoyment by a small party (multiple game controllers, amplified audio, maybe stereo glasses or/and small DLP projector). Load it with information about the country you are visiting, including foreign language aids, some books, music, and movies in local languages if possible. Perhaps, you can master some locally popular board, table, or field game. Depending on the age of your gonna be friends, I'd bring a bunch of small but useful or entertaining gifts to give out. Tiny MP3 players with earphones, small multi tools, keychain flashlights, fishing lures, pocket games - what comes to mind. Perhaps, collectors items like post stamps, postcards, coins... If not directly for your friends - then for their children. Avoid USA symbolics on your gifts though, in many developing countries that might cause troubles if not for your then for your new friends (however, in some countries that could surely have a totally opposite effect to your benefit).

At any rate, learn the region's culture and needs. Perhaps, you could master a sustainable skill or two which are in high demand there, so you can trade it for some local amenities or obtain personal preferencies. Take with you all the necessary tools to efficiently practice them for locals.