The place
Mount Meru, in short.
Meru is what is left of a volcano that blew its eastern side out. You walk up through the gap, along the rim of the horseshoe that remains, with the ash cone below you on one side and a drop of about fifteen hundred metres on the other.
It is climbed in three or four days from Momella gate and it is not a gentle walk. The gradient is consistently steeper than anything on Kilimanjaro, and the summit ridge is narrow. What it gives you in exchange is a mountain with almost nobody else on it, and a first day spent among giraffe and buffalo rather than porters.
An armed ranger walks with every group. This is not ceremony: the lower forest holds buffalo and elephant, and the ranger is the reason the lower slopes can be walked at all.
Most people who climb Meru do it as acclimatisation before Kilimanjaro. It works, and it works better than an extra day on the bigger mountain. Leave at least two days between the two.