The Tongoni Ruins are the remains of a 14th-century Shirazi-era town about 17 km south of Tanga — a collection of coral-stone tombs and the foundations of a small mosque, set in a quiet grove of baobab and palm overlooking the Indian Ocean. The cemetery is the most striking feature: forty or so cylindrical pillar-tombs, some surviving to head-height, decorated with shallow geometric carvings and traces of coloured glazed bowls that were once inlaid as ornament.
A caretaker will show you around for a small donation; allow an hour. The site is reached down a rough track off the main road south to Pangani, or by chartered taxi from Tanga. Pair it with a swim at the nearby beach.