I live in Mwanza, in north-eastern Tanzania, and my brother comes and visits in september. He will land in Nairobi and from there get a bus.
But the only buses I found leave in the evening, while he lands at 5 am...
Thus he's got one day in Nairobi. What interesting stuff can he do there? Is Nairobi NP worth a visit? (we're going to serengeti and ngorongoro after that...)


Nairobi NP has zero interest if you're going to "real" parks after that. it has few animals and always with a view on dowtown skycrappers in the background, which is far from romantic.
What he can do : there is classic touristy stuff like the giraffe center (you feed the giraffes, which is nice), David Sheldrick elephant orphanage (three babies elephants and 300 shouting tourists blocking the view), Karen Blixen House (remember "Out of Africa" ? Nice house but the visit does not exceed 20 minutes).
He can also chose to relax by having lunch in the Talisman restaurant in Karen, in a beautiful swahili decoration and a nice garden. This is a great place full of expatriates and white Kenyans. For more touristy stuff, there is Carnivore restaurant with its platters of meat but far too crowded with touristic groups to my taste.
Not a lot to see apart from that. Nairobi is a nice city to go out and socialize, but there is alsmot nothing to see. Dowtown holds zero interest during the day, most (rich) people go to shopping malls such as Village Market or Junction (nice Italian restaurant there).