The combination works because the two halves ask different things of you. The Mara wants early mornings. Diani wants you to forget what day it is. Put the safari first. Skin that has already had four beach days does not enjoy a 6am game drive.
Fly. Wilson or the Mara airstrips to Ukunda is the hinge. The road from the reserve to the coast looks clever on a map and feels like a wasted holiday. We price the flight in the quote so it is not a surprise at the end.
On the coast, choose a villa if you are more than two, a small hotel if you want staff and a reef boat on call. Say whether you want a pool, a cook, or a town you can walk to at night. “Beach resort” is not a specification.
Pack two bags in your head: dust layers and one decent shirt for the bush; linen and a rash guard for the reef. Leave the formal shoes in Nairobi.