Take the Tokyo Aqualine from just past Haneda Airport in Kanagawa Prefecture under Tokyo Bay, all the way to Kisarazu, Chiba, on the Boso Peninsula. This engineering feat connects the two prefectures with the worlds longest underwater car tunnel. At mid-point you can surface to a man made island in the middle of Tokyo Bay called Umi no Hotaru (sea firefly). Recommend the view here at night time on the drive back.

From Kisarazu head around the coastline in the Tateyama direction. This road will wind around the coast showing you beach after beach along the way. When you get just past Chitose on the outside of the peninsula you can head inland and cut across the peninsula back to Kisarazu for your return trip. Stop at some of the beaches and enjoy some seafood along the way.