James W.
Yelp
What makes this museum four rather than three stars is the early twentieth century building it's in (okay, the mismatched mid-century brutalist annex is yucky but....) There's a pretty cool stained class light fixture inside the entrance facing Bashamichi and the hallway from there around to the entrance to the museum has lots of photographs of Yokohama's twentieth century architecture, many of which haven't been torn down by developers (yet).
The museum proper holds a diverse collection of artifacts from Kanagawa prefecture, from Jomon and even pre-Jomon era pottery and spear tips to samurai armor and any number of Buddhist statues and such.
I wouldn't go out of my way to come to this museum over any other, but if you're in the area and looking for something to do, it's not a bad place to spend your 300 yen on. .... And if you don't want to spend that 300 yen, you can always just pop into the building and wander around the ground floor as long as it's open.