Light, wash and wear clothes will give you the most use. A warm sweater or pullover may be helpful. While the coast is hot year round, Nairobi and the western highlands can be cool at night in July and August. A light rain coat is advisable. On safari, long pants and long sleeves can help for those sensitive to strong sun.
Dust is prevalent, fortunately many camps offer laundry service.
Some hotels have pools, so you may want to include a swimsuit. Good, comfortable walking shoes are adequate for most situations, hiking boots for trekking in mountain areas. Many travelers pack more than they really need. Try to bring only the minimum clothes necessary. In Muslim areas including the coast, be aware of the prevailing customs.