Mount Meru is often overlooked next to Mount Kilimanjaro, but it is definitely worth climbing! It is a beautiful climb, takes less time than climbing Kilimanjaro (and thus less expensive), so it is the perfect climb if you are traveling on a budget, but looking for a mountain climb.
With 4566m Mount Meru is the second highest mountain of Tanzania (if you count the two summits of Kilimanjaro as one mountain). It is a dormant volcano, so on the way to and on the summit you have a spectacular view over the crater, on Mount Kilimanjaro and when it's not too cloudy, over Tanzania. When you are lucky you can spot some animals on the way, because Mount Meru is located in Arusha National Park.
You can choose to climb the mountain in three or four days. The difference of one day is only in the descending. There are two huts on the way to overnight.