After visiting Nikkō National Park, we did a two day summer road trip through Gunma Prefecture, visiting Lake Onuma and Akagi Shrine, beautiful Kakumanbuchi, Fudō Falls and Fukiware Falls, Mizusawa Kannon-ji Temple, Haruna Shrine and Mount Haruna. We stopped along the way at inviting hot springs and road stations with cheap, delicious fruit and vegetables. We ended our trip at Ikaho Onsen. My top three picks were Haruna Shrine (I liked it even better than Tōshō-gu at Nikkō), Kakumanbuchi (natural beauty), and Fukiware Falls (surreal river gorge).
These places will probably be even better in autumn. Need to hire a car?