• 1 day Arusha
• 1 day Ngorongoro Crater
• 3 days Serengeti
• 1 day Tarangire
• 1 day Arusha

Climbing safari:

• 1 day Arusha
• 2 days Oldonya Le'Ngai
• 3 days Mt. Meru
• 1 day Arusha

(Climbing Kilimanjaro with only a week in Tanzania would be very difficult and probably not much fun)

Two Weeks

Game parks and beach:

• All of the above game park safari plus,
• 4 days in Stone Town, Zanzibar
• 3 days in Nungwi, the North shore of Zanzibar

Climbing safari:

• All of the above climbing safari plus,
• One week relaxing in Zanzibar (see above) or
• One week in the game parks (see above) or
• 7 days Kilimanjaro climb ("Machame Route")

Three Weeks

Any combination of the following would be excellent.  However, I would suggest that you finish your journey with a week in Zanzibar.

• One week in Northern game parks (see above)
• One week climbing Kilimanjaro
• One week climbing Oldonya Le'Ngai and Mt. Meru
• One week in Selous Game Reserve (dry season only)
• One week in Zanzibar

Four Weeks

This is the ideal amount of time to spend in Tanzania.  Unfortunately, it's very difficult to be on vacation for this long.

• Begin in Arusha
• 2 days in the Maasai lands, camping in a village
• 1 day climbing Oldonya Le'Ngai, the Maasai "Mountain of God"
• 2 days Ngorongoro Crater
• 3 days Serengeti
• 2 days Tarangire
• 3 days climbing Mt. Meru
• 1 day rest in Arusha
• 8 days climbing Mt. Kilimanjaro ("Lemosha Route" or "Machame Route")
• 7 + days relaxing in Zanzibar

Six Weeks

If you can spend this much time in Tanzania, then you should be able to do everything in the 4 week itinerary, plus head down South to see the Selous and Ruaha parks.  If you are really adventurous, you could try to visit the Chimpanzee reserves (Gombe Stream and Mahale Mountains National Parks) on the shores of Lake Tanganyika, made famous by Jane Goodall.