I know what you're about to say: "But, I've already been to the big buddha at Kamakura. Why do I need to go here?"
I don't always push for my favorite hidden gems, but when I do, they involve magical experiences with a 31-meter buddha (twice the size of the one at Kamakura!), plum blossoms and a somewhat rigorous hike.
Let me backpedal a bit, Sawtooth Mountain ("Nokogiriyama" in Japanese) is tucked away on the peninsula of Chiba Prefecture. It's not as popular with visitors, as it really isn't in a touristy area, but there is plenty of access to Sawtooth Mountain by train, car or even ferry to and from the other side of Tokyo Bay in Kanagawa Prefecture. It has its own ropeway (open 9 a.m. to 5 p.m.), but the hike up the mountain really only takes 40 minutes to an hour. At the top, you get a beautiful view of the coastline, a venture out onto the "sawtooth"-shaped cliff and a visit to numerous historical and mysterious stone-carved statues. That includes the most impressive buddha I've ever seen which is at Nihon-ji, a buddhist temple tucked away in the mountain.
My recommendation is to absolutely go in spring when the plum blossoms come out -- that matched with a blue sky is completely unforgettable. But don't take my word for it... Try out the day trip yourself.
Total time: About 2 hours.
Take a bus from Tokyo Station: Go to the South Yaesu Exit, at bus stop Tokyoekiyaesuguchi to Kimitsuekiminamiguchi. Walk one minute to Kimitsu Station and get on the Uchibo Line for Tateyama to JR Hama-Kanaya Station. Then walk 12 minutes to Nokogiriyama Ropeway.