Kenya's longest river, called the Sagana where it drops out of the Aberdares about 90 minutes north of Nairobi. That upper stretch is the closest white water to the city and a standard Nairobi weekend: half-day rafting on Class III and IV rapids, run year-round from camps near Sagana town, with kayaking, bungee and riverside cottages alongside. Search for Sagana, not Tana, when booking.
It also has many banks built along it, which are architecture and engineering marvels. If you visit the Tana Delta in the Coast province of Kenya, you may also be able to see and visit the Pokomo and Orma tribes. Watching the sun rise over river Tana is a must-have experience.