Clothes for safari should be light, comfortable and not too brightly colored. There is little or no requirement for anything formal. A light jacket or sweater may be needed in the evenings at higher altitudes, comfortable walking shoes are an essential, as is a hat to keep off the sun, and sunglasses. Don’t forget a swimming costume for use at the lodge pools.
Remember that safari vehicles and light aircraft have tight limits on the size and weight of luggage carried. Depending on your itinerary it may be possible to leave some luggage in your hotel in Nairobi and Mombasa for the duration of your safari.