Nairobi and the central highlands are hot by day, cool at night and receive practically no rain.
Mombasa and the coast is hot by day, relatively cool at night, and receives a fair amount of rain.
The Rift Valley and western interior are hot by day, cool at night and receive a moderate rainfall.
September is a good time for beach holidays on the Kenyan coast, being relatively cool and dry.
Game viewing in most safari destinations is good in September. The wildebeest migration is usually well ensconced in the Masai Mara, making it arguably the best time to visit this reserve.
Humpback whales are often seen off the coast over August-September. Being relatively dry in the highlands, September is one of the best months for climbing Mount Kenya.