Apart from Kilimanjaro, the dominating peak in the region, Mount Meru, the second highest mountain in Tanzania at 4,5685m, is a sight that is hard to miss. Its history has been quite explosive. It has had four eruptions, the last of which occurred in 1910.
Various tour operators will offer treks up to the summit. The treks are usually 4 days (hotel to hotel). Besides the stunning views of the landscape, you also can see lots of wildlife en route.