Oedo-Onsen-Monogatari 大江戸温泉物語

Ooedo-Onsen-Monogatari was opened in 2003. It was the first onsen (hot springs) theme park in Tokyo.

At Ooedo-Onsen-Monogatari, the biggest attraction is two of their baths (I believe one for men, and one for women). The baths are have the brimming water pumping out from a natural thermal hot-spring which is located at 1,400 meters below the ground.

Apart from hot springs, a large variety of food and entertainments are also available in side the facility, i.e. nice restaurants, traditional amusements such as fortune-telling, quoits, blowgun darts, etc., and Japanese-style full-body and foot massages.

For relaxation, I prefer Yumegurimangekyo 湯巡り万華郷. It is cosy and spacious. However, you've got to prepare yourself in an alien town, whom the people are 99.99% non-English speaking.

p.s. The food choices are better, and the staff are mostly English speaking in 大江戸温泉物語. Of course, fees and services are more expensive, too.

source: Oedo-Onsen-Monogatari