1. The Inbal Spa — Liberty Bell Park, Jerusalem
The most consistently good hotel spa in Jerusalem, and open to non-guests.
The Inbal's spa is the one Jerusalem residents book when they want a treatment rather than a hotel stay: a full treatment menu, a proper indoor pool, sauna and steam, and therapists who have been there long enough to be asked for by name.
Day packages bundle a 60- or 90-minute massage with pool and wet-area access for the rest of the day, which is the sensible way to buy it — the facilities are a large part of what you are paying for.
It is strictly kosher and closed over Shabbat, and it books out around the chagim.
Good to know: Book the day package rather than the treatment alone — the pool time costs almost nothing extra.