Lake Manyara is really two lakes. Well, it is depending on the time of year you visit. In the dry season, with the water level low, it basically splits in two.
The northern end, where the Simba River feeds in, is swampy with fresh(-ish) water. There are reeds, plants, and lots of wildlife. Hippos soak in the muddy water, and zebras, wildebeests, and impala graze.
The other end looks very different: a vast, flat, featureless plain from the alkaline water. Lake Manyara is, after all, basically a salt lake (or, more properly, a soda lake). The grass, such that there is, is low. Only a few giraffes and wildebeest wander through the hazy heat.
Ernest Hemingway called Lake Manyara "the loveliest [lake] ... in Africa." The National Park, which is relatively small compared with the nearby Tarangire National Park and very different from Ngorongoro Crater, also nearby, is famous for its climbing lions and troops of baboons. You'll see plenty of baboons, although the lions are harder to catch a glimpse of (as are the leopards). And the thick jungle near the park entrance, is the ideal habitat for blue monkeys and hundreds of varieties of birds.
Photos of Lake Manyara National Park
What To Know Before You Go
Travel Advice for Tanzania
You can find the latest U.S. Department of State travel advisories and information for Tanzania (such as entry visa requirements and vaccination requirements) here.
Health & Vaccinations
The CDC makes country-specific recommendations for vaccinations and health for travelers. You can find their latest information for Tanzania here.