Both my brother and my sister-in-law were in the Peace Corp in Africa (two different countries). Like someone else said, you cannot count on the avalability of electricity and I would add computer/Internet and phone. And if is available, it may only be available at certain times of the day. So I would be careful about giving them electronics that need to be charged and probably not anything that needs a constant source of electricity. My brother lived for two years with no electricity or running water.
I also wouldn't give them too much that they would be torn about leaving behind. After living there for two years they will have accumulated so many things that you can only get in Africa. When my brother came back to the States he really had a hard time deciding what to leave behind. He actually ended up leaving behind almost everything he had originally taken with him.
The things he appreciated the most were the things that be could not get in Africa--food items, pens/markers, personal care items, new reading materials, socks. I like the idea of a really nice journal for each family member and the promise of care packages. Once they get there they can let you know what they miss and you can send it to them. A friend of mine who is a missionary loves to get her hometown newspaper, recycled magazines, and seasonal treats like jelly beans.
A couple of cautions about care packages to Africa. Postage can be very high, so don't promise a monthly package unless you are sure it fits into your budget. Packages are likely to be opened by customs agents and things may be taken, so don't send something of high value. There are some items that may have import taxes applied to them for example fabric. So be sure to check on that. Finally, be honest on the customs forms you fill out at the post office. If the box is opened and they find discrepancies the entire box may be confiscated.
The best gift you can give them is the gift of daily prayer!