Kisite-Mpunguti Marine Park and Wasini Island is the best day trip from the south coast by some distance. Tours depart early from Diani, drive south to Shimoni (about 80 kilometres), then take a dhow out to the reef for snorkelling and dolphin watching. The afternoon is spent on Wasini Island, with a seafood lunch at Mpunguti Restaurant included in most packages. The day is long but the combination of reef, dolphins, and a vehicle-free island is hard to beat.
Shimba Hills National Reserve works well as a half-day trip from Diani — close enough for a morning game drive and back for lunch. The reserve is small but the sable antelope are unique to this location in Kenya, and the forest elephants and canopy walk add variety. Most Diani lodges can arrange the trip.
Sheldrick Falls within the Shimba Hills can be combined with the reserve visit — a short hike leads to a cascade in the forest. The trail is easy enough for most visitors and the forest is cool and shaded.
Mombasa makes an easy half-day trip. The Likoni ferry takes five minutes and the old town with Fort Jesus is worth a couple of hours. Allow more time if you want to explore the markets and the Portuguese fort properly.