The Ngorongoro Crater is normally done as an overnight from the rim, but it can be reached and explored in a single long day from Arusha — roughly 3.5 hours of driving each way, plus the descent and a game drive on the crater floor. Expect an early start and a late return.
The enclosed crater has one of the densest wildlife concentrations in Africa, so even a half-day on the floor gives a realistic chance of lion, elephant, buffalo, hippo and — with luck — black rhino. If your schedule allows, the two-day version (with a night on the rim) is far more relaxed.
Very early departure from Arusha. Drive to the crater rim, descend for a game drive on the 260 km² floor with a picnic lunch, then ascend and drive back to Arusha, arriving in the evening.