Oze is a famous mountain wetlands area located at the shared border of Gunma, Niigata, Fukushima. The lengthy wooden pathways are well maintained and hiking in Oze can be really refreshing. It is a nice place to spend 2 or 3 days, but a one day trip from Tokyo is not out of the question.
https://www.env.go.jp/en/nature/nps/park/oze/index.html
https://www.pref.gunma.jp/04/e23g_00065.html
https://www.japan-guide.com/e/e7440.html
