Updated:
Spa Nescens at the Victoria-Jungfrau offers Spa Days for people staying elsewhere. If you are already spending a day in Interlaken, book a few hours for the pools and rest areas. A treatment can be part of the visit, but check which spa facilities come with it.
An enquiry is only the first step; wait for the spa to confirm a reservation. Day guests have their own access hours and age rules. When choosing a package, ask which pools, saunas and relaxation areas you can use.