There are two near me in cities:
Bernaqua: Rates for 1.5 hrs, 3 hrs, or all day. Pay to park. Waterpark included in admission (and they have some great slides!).
Yverdon-les-Bains: 3-hr entrance. Free parking. The saunas and the outdoor jacuzzi are extra (but the jacuzzi is worth it at sunset if there aren’t a lot of people).
For the most part, thermal baths are found in the Alps, near hot springs. Here are the ones I like or have heard good things about:
Anzere Spa and Wellness: Can pay for a 2-hr or 3-hr entrance. Aquagym included in admission (sessions M-F 5pm, Sat 1pm).
Bains de la Gruyeres at Charmey: 3-hr or half-day entrance. Free parking. Saunas and hammams included in admission. Best after a day of sightseeing in Gruyeres (Cailler chocolate factory, Gruyeres cheese factory, castle, town).
Lavey-les-Bains: Can pay for a 3-hr or 4-hr entrance. Free parking. Saunas and hammams included in admission. This thermal center has bragging rights; its water comes from the hottest spring in Switzerland.
Leukerbad: There are actually two spas here, Leukerbad Therme (+waterslides) and Walliser Alpentherme & Spa. I’ve only been to Walliser Alpentherme, which has a 3-hr or all-day entrance. Pay to park. Aquagym classes included in admission. It has my favorite mountain view from the outdoor pool. If you want more privacy, top-knotch hotels here also have thermal baths just for their guests.
Ovronnaz: Bathing time is unlimited (as long as you don’t leave). Famous for their ski/spa package.
Saillon: Bathing time is unlimited (as long as you don’t leave). Saunas and hammams included in admission.
Therme Vals: Reservation required. Total relaxation.
Val d’Illiez: 3-hr entrance. Pay to park. Sauna and hammams included in admission. The building itself is beautiful; if they had oriented it differently, the view could have been spectacular.