Pemba is a coastal town at the mouth of a huge bay. It boasts some interesting buildings (especially in Baixa - the old town) and a lively atmosphere. Most visitors come for the beaches - particularly Wimbi Beach - and the coral reefs.
Wimbi is about 5km (3m) east of town. The reef is so close you can swim out to it. A fledgling tourist industry is already transforming the place with bars, restaurants, diving, snorkeling, boating, fishing, etc.
The Makonde workshop on the road between town and the beach has good wooden carvings for sale at even better prices (but stay away from the ivory). Bus or hired car is the way to get there, though you can get a dhow by asking around the port.
The best time to visit Mozambique, is the months of June to August, when both rainfall and temperatures are at their lowest. Later in the dry season, in late August and September, the temperatures start to climb, but this is the best time to see big game. The best time for bird watching, on the other hand, is usually November and December.