An onsen, or hot springs in English. A nice place for your sims to come and relax. (If you want to keep with real tradition, however, you'll need a mod that lets sims get in the hot tub naked!)
Requirements: Nightlife and Celebration Stuff!
Also, even if you don't get any of the other custom stuff, you MUST MUST MUST get this or I will come to your house and rip the lot out of your hood! Thank you so much reica15!