The place
Tanga & Pangani, in short.
This stretch of coast has the history of Zanzibar and almost none of the visitors. Tanga was a German and then British port built on sisal, and the town still reads that way. Pangani, at the mouth of its river, was a Swahili and Omani trading town deep in the slave and ivory trade, and it is now very quiet indeed.
The reason to come is Saadani. It is the only national park in Tanzania that fronts the Indian Ocean, and elephant, buffalo and giraffe genuinely do use the beach. Green turtles nest at Madete. The Wami River gives you hippo, crocodile and a good chance of the coastal birds, from a boat.
Inland the ground rises quickly into the Usambara Mountains — cool, forested, full of species found nowhere else, and walked from Lushoto and Amani.
None of this is polished. The roads are mixed, the choice of places to stay is short, and that is precisely why it is worth the trip.