The North Sumatra Museum is Medan's best broad introduction to the province, especially if you are heading onward to Lake Toba or other parts of the region. Its displays bring together archaeology, ethnography, colonial history, and material culture from the many communities of North Sumatra.
It is a practical museum rather than a flashy one, but that is part of its usefulness. A visit gives names, objects, and context to the Batak, Malay, Chinese, and Islamic strands you will keep meeting around Medan.