Mostly Nairobi and Mombasa with a bit of time at Nakuru. Mombasa is about 8-12 hr bus ride from Nairobi and is a beach city. Nakuru is about 2 hours from Nairobi and is a medium size city with a couple of lakes and safari areas nearby.
When: first quarter of the year. Not too hot or humid and hardly any rain.
Pre-trip info: mostly gleaned from ISG, a little from GGC and a bit from some monger emails and PMs. Some guide info from Lonely Planet.
Getting there: from the USA you will be hard press to find a deal. The best I saw was $1300. This makes the destination a bit hard to justify when better deals to Asia are much more easily found, without mentioning some good south america locations. You can choose to fly via a number of euro cities, most common London and Amsterdam (will pick the second next time!). However, if I were based in Europe I would not give it a second thought to fly down here for some R&R.