If you have limited time in Nairobi then you can't do better than to find the private car hire firms on the upper levels of the exit hall and offer the boss to drive you around for the day.
for the same price as an avis hire car we were driven where ever we wanted to go for 12 hours. Our driver gave us a wide selection of sights to see and then mapped out a route and took us there. Fantastic way to see the things that the locals consider special in a very short stop over.