The Kenyan coast has a different history from the interior. Swahili city states traded across the Indian Ocean with Arabia, Persia and India from the ninth century, producing a coastal culture with its own language, architecture and cuisine long before anyone came inland.
Mombasa
Kenya second city and its main port. Fort Jesus, built by the Portuguese in 1593 and fought over sixteen times, is a UNESCO site with a good museum. The old town behind it has carved doors, balconies and narrow lanes, still occupied and best walked with a guide who can explain what you are looking at. The city itself is hot, crowded and less pleasant than the beaches on either side.
Diani and the south
Diani Beach, south of Mombasa across the Likoni ferry, is the best-known stretch of white sand in the country: 17 km of it, backed by coral rag forest with colobus monkeys, and protected by a reef that keeps the water flat. Tiwi Beach next door is quieter and cheaper. Shimba Hills National Reserve inland has sable antelope and elephants and is an easy half day.
Watamu and Malindi
North of Mombasa, Watamu has a marine national park with excellent snorkelling among coral heads, turtle nesting beaches and a sheltered bay. Malindi has a long Italian connection and the food to prove it. Between them, the Gede ruins are the remains of a Swahili town abandoned in the seventeenth century for reasons still unclear, now swallowed by forest and completely atmospheric.
Lamu
The oldest continuously inhabited Swahili settlement, a UNESCO World Heritage site, and a place with almost no motor vehicles: transport is by donkey and by dhow. The town is a warren of coral stone houses with carved doors and inner courtyards, and life follows a rhythm set by prayer and tide. Shela village has the beach. Lamu is reached by flight from Nairobi or Mombasa; the road through Garissa is not recommended.
Practicalities
The Madaraka Express railway runs Nairobi to Mombasa in under five hours and is comfortable and cheap. The coast is Muslim-majority and modest dress away from resorts is appreciated. The best months are December to March and July to October; the long rains in April and May close many places. Malaria risk on the coast is significant year round.