I was in Kenya quite a few years ago, so my situation may be different from the current. But here goes . . .

Safari's run the gamut from basic (tents and barbecue cooking, usually at a camp with communal men/women's bathrooms with hot showers) to extravagant (see above). Your best option is to get there and check which tour operators (there are a gajillion in Nairobi) offer what you want. I was there for studies, but squeezed in 2 safaris of 3-4 days. The first one was a splurge (for a student): at the Masaai Mara, the camp had 'tent's with beds and attached private baths, swimming pool, and gourmet food. The second was more budget: at Amboseli, with puptents, open grilled food and the communal baths. Both were fun -- obviously the first was easier on the back but not on the wallet. Sorry, I do not recall the prices, but they are sure to be much different by now.

As far as where to go -- if you stay in Kenya, Masaai Mara is still the best to view animals; Amboseli and Tsavo have been decimated, so I hear. Mt Kenya and "Treetops" hotel is interesting, different, but not sure of the animal-seeing factor. My impression, though, is that Tanzania has done a better job of preservation, and most animals have migrated (and survived) south of the border.

Besides Nairobi, I would definitely head for the coas for at least a few days. The coast has a definite Arabian flavor and was settled much before the Brits inland. Mombassa is a big city but has interesting Portuguese and Arabian sites. I spent a few days up the coast in Lamu -- pretty idyllic, a small town right out of 'Arabian Nights'. Not much to see or do, except a bit of shopping, bath on the Indian Ocean, and soak up the culture. There are flights to Lamu and Mombassa, pretty cheap -- they leave from the domestic airport, Wilson. The other major coastal town to consider is Malindi.

PS -- if you are thinking of a stop-over, Cairo is not a bad idea. Although you cannot do the city justice in a few days, it is too facinating to pass up if you are in the area. Between the ancient Egyptian, Roman, and Islamic periods, there is much to see.