I went in Tanzania in October 2002.
Travelling by yourself in Tanzania is easy and cheap : most part of city people can speak english and life, in the Tanzanian style, is really cheap.
In Arusha, a lot of travel agencies can fix you whatever kind of tour you want.
Costs span a great range, benefits too. Every one can make its own choice avoiding the agencies in the dark list.
This dark list is available in the touristic office in Arusha.
Day 1 : Paris - Arusha (International airport of Kilimandjaro)
Day 2 : Arusha (fixing next tour)
Day 3 : Arusha - Kilimandjaro climbing
Day 4 : Kilimandjaro climbing
Day 5 : Kilimandjaro climbing
Day 6 : Kilimandjaro climbing
Day 7 : Kilimandjaro climbing - Arusha
Day 8 : Arusha (healthing and fixing next tour)
Day 9 : Arusha - Serengeti
Day 10 : Serengeti
Day 11 : Serengeti - Ngorongoro
Day 12 : Ngorongoro - Arusha
Day 13 : Arusha - Zanzibar (flight)
Day 14 : Zanzibar
Day 15 : Zanzibar - Dar-Es-Salam - Paris