Lake-filled Empakaai Crater, 23km northeast of Olmoti Crater, may not be as famous as Ngorongoro, but many travelers consider it to be its match in beauty. The lake, which draws flamingos and other waterbirds, fills most of the crater floor, which is surrounded by steep-sided, forested cliffs at least 300m high. The view from the crater rim is one of the most appealing in northern Tanzania, but hiking down into the crater is a wonderful experience as well.
The road from Ol Doinyo Lengai runs along part of the eastern crater rim, which varies in altitude from 2700m to 3200m, and a steep, well-kept trail descends from the road through montane forest rich in bird life; also keep an eye out for hyenas, buffaloes, blue monkeys, and even elephants. En route to the crater floor, watch for views of the turquoise lake down below and the perfect volcanic cone of Ol Doinyo Lengai away to the northeast. Count on around 30 minutes down to the lakeshore, and an hour for the climb back up. It's possible to circumnavigate the lake on foot, which will take at least four hours.
If you're planning on hiking, you are, in theory, required to pick up an armed ranger (US$23.60) from the ranger post at Nainokanoka (next to Olmoti Crater) en route to Empakaai. To get here, count on a 90-minute drive from the Lemala ascent-descent road.