The Nairobi National Park is within 20 mnts drive of the airport opens at 5.15 AM to 5 or 6 PM and can provide very nice views of wild animals from baboons and gazelles to lions and giraffes. The problem is whom to go with and at what time. Do not go with touts or soliciting drivers (VERY dangerous anything can happen as you'll be alone on thousands of acres). If you cannot arrange ahead for a professional guide to meet your flight go to the Kenatco window outside customs rent a taxi by the hour 20$/hr plus entrance fees and tip should be accepted). Just be aware that you'll need a visa and you'll pay the airport tax. Also while dawn and dusk are very good there is little to see during the day everything sleeps. Without an experienced guide you anyway most likely won't see anything dramatic. And it will be hot. Take water.