Northern Tanzania is famous as one of the world's top safari destinations: the notorious Serengeti and Ngorongoro Crater offer excellent game viewing all year and is also home to The Big 5 and The Great Migration. It is the perfect destination for first-time safari-goers and experienced safari regulars. The only drawback of the North is that because it is famous, it gets busy, and costs are a little higher due to high demand. Activities here are also regulated; most lodges don't offer anything other than a standard game drive (no walking, boating or night drives).
Southern Tanzania offers a different experience. The diamonds of the South are Selous Game Reserve (the largest game reserve in Africa) and Ruaha National Park - both have stayed pristine by mass the travel industry and subsequently offer a more valid and personal safari experience. The South is in season from early July until October and has fantastic game viewing. The rules here are more relaxed, allowing boating, walking and fly camping in the bush. The accommodation is better value too (more luxury for your money) and the parks are closer to each other and the beach, meaning less time and budget spent on travelling.
Zanzibar is a fantastic option for a beach break. There are many superb beach options to choose from, with prices varying hugely. Whether you're after stripped back rustic experience or a luxury stay, Zanzibar has it. Other surrounding islands cater well to big diving fans (Mafia and Pemba), and a few private islands like Mnemba and Fanjove cater well for those wanting an exclusive experience.