Dolphin and/or Whale Watching
On the south coast at the Choshi Marine Research Laboratories, you may enjoy Dolphin and Whale Watching, some excursions even run several times daily. Courses are two types - either along the coast or further out to sea. However, any children must be over three- or ten- years of age respectively in order to be able to participate.
NB: This is not my photograph, rather from the URL listed below. Though they surely wouldn't mind in this case as I'm helping them with free marketing :-)
Other local businesses offer an assortment of tourist fishing excursions here.