Hakkeijima Sea Paradise is a purpose-built island of aquariums, marine shows, restaurants, and rides on the south side of Yokohama Bay. It is not subtle, but it is one of the city's easiest family outings, with enough indoor and outdoor pieces to fill most of a day.
The aquarium side is the reason to come; the amusement rides are a bonus if you have children or want a breezy bayfront afternoon. It is farther from central Yokohama than Minato Mirai, so check train connections and give the trip enough time to feel relaxed.