Tel: 0466/290-688
Address: 2-1-6 Enoshima, Fujisawa City, 251-0036
Price: 2,650 yen for adults, 1,200 for children.
How to get there:  There are three train lines to Enoshima. Take the JR Tokaido Line to Ofuna Station. Change to the Shonan Monorail, ride it to the end, then walk out to the island.  10-minute walk from Katase-Enoshima Sta. (Odakyu train line) 15-minute walk from Enoshima Sta. (Enoden train line) 15-minute walk from Shonan-Enoshima Sta. (Shonan Monorail line) 
Official site 

EnoSpa, on the small island of Enoshima, is an upscale complex with a variety of hot spring pools less than an hour from Tokyo. They is an array of indoor waterfalls, a health food cafe, whirlpools, cave-shaped pools and traditional onsens. Some of the pools are just for soaking and others you can swim in.  All bathing is shared, except in the more traditional segregated onsens which you can access from the locker rooms.  They also do spa treatment, but reservations are recommended.    They also have the world’s first fake aurora effect!