Yamagata Prefectural Museum is the broad background stop inside Kajo Park. Its exhibits cover natural history, local culture, archaeology, and the wider prefecture, with the Jomon Goddess figurine as the standout object.
It is a good choice if rain interrupts your castle walk or if you are using the city as a base for the rest of the prefecture. The museum gives the mountains, pilgrimage sites, and food culture outside Yamagata City a bit more context.