You are on the Equator, even though the temperature is very moderate, the UV rays are very strong. Wear a hat and sun glasses. Sunscreens are necessary for most people. Drink water or other liquids regularly. Highly perfumed products may at act mosquitoes.
Vans are spacious and comfortable but have limited luggage space.
Some people bring a bag just to carry home souvenirs. This can be left in Nairobi and filled before departure. The roads in Kenya are very rough, so be prepared for some jostling. The scenery and animals are beautiful and interesting, but bring a good book for the down times. They're easier to load into cars when moving camp.