There is no shortage of places for day trips - Nikko, Hakone, the Fuji 5 Lakes area, Karuizawa, Kusatsu, Izu, Kamamura/Enoshima, Kawagoe, Takao, Okutama, Yokohama, the Ushiku Great Buddha, Nokogiriyama, Minakami, Showa Kinen Park, and more. There are regional rail passes you can get for nearly all of them. A Tokyo Wide Pass might save you a bundle.
For hot springs, Kusatsu is a large hot spring resort area, but you could also try Hakone, or by Nikko there is Kinogawa and Yumoto. There are many others in the Tokyo area as well.
Hello,i will recommend to go Hot Spring,Staying Ryokan ,ATAMI ONSEN area,Hakone ONSEN area and Kusatsu ONSEN area,not so far from Tokyo to there .
How about Atami ? From Tokyo, if you take shinkansen, it takes about 30 to 50 mins depends on type of shinkansen. Even with rapid train, you can arrive in 2 hours. Atami is one of oldest and most famous onsen place around Tokyo. Like my parents went there for their honeymoon. :) I have been this summer and I really enjoyed sea, onsen, food and small shopping etc. There are many traditional ryokans and hotels. For your reference, I will introduce one website in English! https://www.atamispa.com/atami_ex/english/list/index.htm Have a nice trip with your family!!