As well as everything else already mentioned, there's also the David Sheldrick Wildlife Trust aka the Elephant Orphanage. You can visit and adopt a baby elephant! If you like reptiles then there's also the Nairobi Snake Park. If you've heard of the movie Out of Africa, then definitely go to the Karen Blixen House and Museum. about 8 years ago
Where is the best area to stay in Rome? (Family of 4, with 2 teenage children) Any hotel recommendations?
With teenagers an apartment might be a better idea than a hotel. I've stayed at the Sunflower House which has studio and two bedroomed flats. It's right across from the Basilica and near Trastevere that has lots of pubs, restaurants and areas to hang out for the teens. Have a look at http://www.sletoh.com/hotel/Sunflower-House-61506.html to get an idea. about 8 years ago
You could try the Lionja Forest Reserve in Lindi Region. about 8 years ago