Enospa in Enoshima
Whether you're a spa enthusiast or first timer, I highly recommend you visit Enoshima's Enospa for an affordable, exclusive experience you won't soon forget.
For less than ¥3000, you can spend an entire day relaxing in one or all of their various indoor and outdoor pools (with beach/ocean view), as well as their sauna. Enospa also provides several cafes, restaurants, and attractions to keep you entertained. And once you're finished, you can enjoy everything else the popular island has to offer.
After having lived in the Shonan area for many years, my only regret was that I'd waited so long to go. But I intend to make up for it by becoming a member and frequenting Enospa regularly!
My fellow sensei, Andrew Grimes, has a wonderfully exhaustive write up (in three parts) here at DeepJapan. I encourage you to read them all to get a better feel of what Enospa is like: