Bwindi
The Bwindi Impenetrable forest on the Virunga mountain ridges that Uganda shared with the Democratic Republic of Congo is a great place in which to see Mountain Gorillas. From Nairobi, you can fly to Entebbe in Uganda and then on to Kihihi inside Bwindi or else fly to Kigali in Rwanda and then drive to Bwindi. There are flights to Entebbe from Masai Mara too. So it is possible to start or end your Kenyan safari with a gorilla safari in Bwindi.
The gorilla trek involves a day of walking in mountainous terrain to see the mountain gorillas. If it has rained it can be fairly muddy as well. There are guides to assist on the steeper stretches but even so. some of the routes can be strenuous. So, a gorilla-trek calls for an element of physical fitness.