Giretsukan is a small museum on the grounds of Tokiwa Shrine, beside Kairakuen Garden. Its collection includes a large rolling taiko drum, an old cannon, Tokugawa family material, and exhibits linked to Mito's late-Edo history.
It is not a destination on its own, but it is a useful stop if you are already visiting Kairakuen, Kobuntei Villa, and the shrine precinct.