If you don't have time to got far from Tokyo, why not to go to
Hotel Emion Tokyo Bay
Spa and Holte Maihama Eurasia
http://www.my-spa.jp/ (sorry only in Japanese)
It takes only 20mins from Tokyo station! And close to Tokyo Disney Land!
My recommend is Kusatsu, Takaragawa, Kamimoku.
① bus 4h
from shinjuku in tokyo to kusatsu
◇ time table
②train and bus, about 3h
Ueno Station → (Approx. 2hours and 30 min by Tokkyu Kusatsu) → Naganohara Kusatsu StationNaganohara Kusatsu Station → (Approx. 25 min by Bus) → Kusatsu Onsen "Hot Springs"
details (Kusatsu onsen tourism association)
Shinkansen (1hr 15min) from Tokyo station to Jomokogen station then the Onsen shuttle will pick you up.You need to book the shuttles in advance.
T akaragawa onsen details
☆ Kamimoku tatsumikan
Shinkansen (1hr 15min) from Tokyo station to Jomokogen station then bus (400 yen).
bus time table
http://www.tatsumikan.com/ Traffic guide /
Kamimoku tatsumikan details
I recommend to buy JR Tokyo wide pass!