Hi! I just came back from 2 months in Kenya and Tanzania (working - unfortunately not herping full time. The region has experienced a drought for the last 2 years and even the rain forest on East Usambara in Tanzania was dry! The lack of rain has had a dampening effect on all herps. I only saw two snakes. Fortunately, I found several species of Chamaeloens on Usambara though. So if you plan to go next year keep an eye on the weather and rainfall situation ( lack of rain has also had the side effect of severe electricity shortage in Tanzania with frequent day-long black outs.
Given that the rains come back to normal I recommend you go during one of the rainy seasons when most species are active and can be most easily found. Bear in mind that the rainy seasons differ duration and mothns in deifferent parts of East Africa.
The Bioken snake safaris at Watamu Beach in Kenya are highly recommende dif they still do them. In general it is very difficult to find reptiles in east Africa compared to many places in the United States and if you dont want to spend weeks on wild goose chases you should have a guide such as those provided by Bioken. In the Amani Nature Reserve in East Usambara (Tanzania) there are also excellent guides who can help you with morning and night excursions.
Another good place to visit in Kenya is the Kakamega forest close to the Uganda border where many interesting forest species can be found at night.
If you have any questions I would be glad to answer them to the ebst of my ability.
Tom