We went for out honeymoon 3 years ago to Tanzania followed by a week on Zanzibar.
We stayed on the southern edge of the island, the name of the ‘resort’ escapes me, I’ll try and find it later. We found it incredibly relaxing, but you had to as there is not much else to do. We were disapointed with the beaches where we were, the tidel flow was so extreme that you had to walk out for about 10mins to get to the sea at low tide across very sharp and dangerous coral. I have heard other people say that the beeches are not quite as nice as the brochures make out. We certainly thought we had been to better in the med.
Stone town is worth a visit. We stayed in a fantastic quirky little hotel… right next to 2 local mosques. We didnt really see much of stone town itself though as we spent 36 hours with severe d&v to the point we nearly ended up in hospital.
One thing I would say is be prepared for the poverty, although one of the richest places in Africa there are still some very poor people, and we actually felt guilty as much as the money we put in to these nice posh hotels, doesnt actually get in to locals hands.