Sunpu Castle Park marks the site where Tokugawa Ieyasu spent his final years after handing the shogunate to his son. The original keep is gone, but moats, stone walls, reconstructed gates, gardens, and excavation areas still make the park the centre of Shizuoka's Tokugawa story.
It is also a good everyday city park, which helps. Come for the history, then linger because the space is open, green, and easy to reach from the city centre.