Costs
Taxis are cheap (around $15 for half an hour) and matatus are even cheaper, but if you will be moving around a lot, think about hiring a car for a day (around $70). Taxis from the airport to town are roughly $20. Not all of Nairobi's taxis are metered, so be sure to fix the price in advance to save arguing when you arrive.

Safety
Use the hotel safe, don't carry more money than you need for the day and don't do anything confidential, like banking, from an Internet café. Don't buy tours or souvenirs from touts, don't trust friendly strangers who approach you in the street with hard-luck tales (like escaping Sudan as a refugee) and don't walk after dark.

Tips
Gratuities are welcome and usually around 10%, but are mostly only expected by porters and in restaurants catering to predominantly foreign clientele.

Health
Fortunately, Nairobi is all but malaria-free, but Kenya's coastal regions and the interior around Lake Victoria are not. Be sure to take proper precautions.

Airport
Leave plenty of time to negotiate Jomo Kenyatta International Airport, where a security check at the entrance means you can queue for an hour just to get in, and the world's slowest officials mean you can lose another hour at passport control. Visas for most nationalities are available on arrival for $25.

Traffic
Do all you can to avoid rush hour, from 7 a.m. to 9 a.m. and between 5 p.m. and 7:30 p.m., when most of Nairobi's main roads are gridlocked.