Arusha national park located in northeastern Tanzania is the best option if you only have time for a day trip, while Tarangire southwest of Arusha, or Lake Manyara Park is each simply visited as overnight trips from Arusha. For a half-week itinerary, try any one of the northern parks on its own, or Ngorongoro crater together with either Lake Manyara or Tarangire. In a week you have more than enough time for the traditional combination of Lake Manyara, Tarangire, Ngorongoro, and Serengeti though it’s much better to focus on just two or three of these. The distances to Ngorongoro and the Serengeti are quite long, therefore this is likely to leave you feeling like you’ve just been rushing from park to park without settling in to experience the real environments. If you’re interested in a northern safari, with the inclusion of Serengeti national park, then plan for a minimum of five days from Arusha to fully appreciate the beautiful landscape. Other possibilities combine wildlife viewing with other activities like vehicle safaris in the Ngorongoro crater, hiking Oldoinyo Lengai, trekking in the Ngorongoro conservation area, or visiting Lake Eyasi. And perhaps a couple of days exploring the Lake Manyara area by bicycle, canoe, or vehicle, with a day at Ngorongoro crater and another doing short hikes in the Ngorongoro conservation area.