If you are of a very high fitness level, and you have begun the day early enough, you can divert to the summit of Mt Ngauruhoe (2287m)once at the South Crater. This is estimated to be a 3 hr return trip to this point. However, not only is this path steep and requiring high level fitness, but it is a scree slope. This entails high level of energy and patience. Often you take maybe 3 steps up and 2 steps back. The scree means your feet sink into it although this can be to your advantage if you know how to use it.
The day we were here, a tourist was injured falling. He tumbled a long way down and sustained head injuries, with concussion and shock and abrasions. Not so nice.
You will need to give yourself probably a two night stopover in this region to do this walk. To get an early start, you need to spend the night nearby and it's a big and long day, so you probably need to stay the second night as well, to help rest and savour your achievement of the trek.
With plenty of time and energy you can make a side trip to the summit of Mt Tongariro. This mountain is a cluster of many volcano craters, and you are essentially walking on it the whole time. But you can visit the summit (1967m) from the main path. From the rim of the South Crater before you descend to the Emerald Lakes, there is a marked poled track to this summit. Allow 1½ - 2 hrs.