I travelled to Kenya in June this year. It would be my third trip here in 12 months, and what an interesting set of journeys they have been.
The very first time, I flew with BA direct from LHR-NBO; the second time was from LGW-Mombasa on Tsunami day, no less, with Brittania; and finally, this last time, I flew with KLM LHR-AMS and AMS-NB0 coming back. This time I flew NBO-AMS with Kenya Airlines and from AMS-LHR with KLM.
What you need to know: The food on British Airways in both directions was wonderful.
Brittania Airlines was rather amusing in that although I hadn't ordered any food from them, they hadn't paid any attention to the fact that the people next to me had ordered specific foods (dietary need) or were vegetarian (Hindus) and certainly couldn't eat BEEF sausages, so I still got to eat a hearty meal at both ends of the journey. However, if I had paid the money, £10, for the food, I would have been extremely angry, as it was overpriced and not enough. On KLM Airlines, on the trips to and from Amsterdam, the food was good enough for such a short trip, but the long haul wasn't up to much! Kenya Airlines had wonderful food.
The People of Kenya are wonderful and nearly always friendly. Do try to speak a few words of Swahili before you get there, and not just the Djambo greeting that seems to be reserved for Wazungu (white) tourists.
With regards to things to do in Kisumu: Certainly visit their museum and the impala park. If you do decide to go to Hippo Point, make sure you don't agree to anything till you see the boat, and be clear that "no" means "no", despite the persistence. It's probably best to use the bicycle taxis to get there; then you've got an ally, if necessary (I had the advantage of my Kenyan fiancee).
Mombasa: You have to go to Fort Jesus and the Old Town, but make sure you have a good guide with you.
In both Kisumu & Nairobi go to the museums, excellent value for money. If you intend going back more than once to any of the major towns, get the taxi driver's mobile number (they all seem to have cards for this purpose -especially In Mombasa). Then you have no problem getting about. Use the matatus if you want, sometimes it may be necessary.
BUT THE MOST IMPORTANT THING IN KENYA-RELAX & SMILE. IN NAIROBERRY BE ALERT, BUT STILL SMILE & RELAX!