No road, no trip
Sad to say, not every inch of Kenya is covered with neatly processed asphalt. The major towns are linked with good roads for the most part. However, apart from the main roads, few are in the best of conditions. When planning for a road trip, always do some little research. Plan on what road you will use. Find out the traffic activity. Roads away from major towns usually have little or no traffic. For example, if you plan to go to Kericho and don't desire to stop frequently, the best road would be the Mai Mahiu-Narok-Kericho route that has little traffic. A good road will help you choose a good car too.
It would be no use to obtain a 4x4 for a trip from Nairobi to Nakuru or Nairobi to Thika. The roads are good all the way. However, if you plan to go to Busia or Kisumu, you may face a few rough patches that require a great car. A road trip in Kenya without a serviced car is the first step to ruining that road trip. Kenyan garages are not open 24-hours a day. If your car breaks down at dusk, you may be lucky to get an open mechanic store before morning. Service your car before the trip and ensure it has no recurrent problem. A spare wheel and knowledge to fix a puncture will be helpful.
You may need to refuel your car or grab a snack for yourself. Avoid credit cards. They are hardly used outside the cities. Carry solid cash. An alternative would be to carry your exchange on mobile-money systems. The mobile money centers in the country are numerous and you can always withdraw from an agent or pay directly using your mobile phone. Foodstuffs in rural areas in Kenya are very affordable too. Your money can save you lots of grocery shopping. For example, you can buy seven heads of juicy pineapples at only Kshs.200 in Bomet.
Central Kenya is cold in midyear. I travelled to Western at the same time in 2013 and was quite surprised to find it hot all day with light showers in the evening. The weather forecast of the neighboring county might be of good interest to you in order to select what clothing to carry. Places like Sacho in the Rift Valley can be very cold and rainy in December while the rest of the country experiences general warmth and sunshine. Find out the weather of your destination at that time of year.
Food and Water
Highway restaurants are generally full especially during peak season when the children are home for holiday. Packed meals will save you travel time. Many travelers fear food poisoning from eating in new places. Generally, Kenyan restaurants are clean with good food. However, if you have certain preferences and certain sensitivities, packed food would do you good.
I have travelled to Western Kenya every 24 years of my life and I never tire taking photos of the Rift Valley escarpment. I see Mount Longonot and it is as if my eyes have laid sight on the picturesque scene for the first time. Even if you live in Oloitoktok, the towering majestic Mount Kilimanjaro still holds a bewitching hold over your eyes. There is no better way to feast on this beautiful country than seeing the places and taking the photos. Carry a camera, always.
And last but definitely not least, carry an adventurous spirit for the people of Kenya are among the friendliest folk you will ever encounter, south of the Sahara.