Which are the main spa baths in Switzerland and which are their characteristics?

I'm not sure about spas, but if you're interested in hot baths, there are quite a few in the lake Geneva area (the area I'm familiar with):

(Note: I've only ever been to the first two)

Ovronnaz

Up in the mountains, astounding view, surrounded by mountains. In the winter, you can be outside, up to your neck in steaming water, with snow falling on your head. This place is child-friendly (they let them in, and under-sixes for free, and there's a special toddler paddling-pool), and non time-limited. It's quite a drive up the mountain to get there. The baths are not too expensive in themselves, but accommodation, food etc., is not particularly cheap 'up there'

Main selling point is that you can combine the slopes and the hot baths. One of the best days out I ever had was sledging followed by hot baths followed by a fondue aux bolets!

Lavey

More accessible, more things to do - whereas at Ovronnaz there are 3 basins (one inside, two out) and a 'jaccuzi', at Lavey there is also Hammam, Sauna, Turkish baths, etc, etc. The water is naturally hot here - they actually have to cool it before putting it in the baths, I believe... Here you have the full hotel/spa/wellness 'thing'.

Time limited (3 hours, by default), no babies or toddlers (4+), expect queues on weekends with bad weather...

Val d'Illiers

Never been, recently (end 2010) refurbished/reopened. Also time-limited and no babies or toddlers. Looks like they have the full spa/sauna/wellness package too.

Gruyère

Fairly new, time limited (3 hours, by default), no babies or toddlers (4+). The panorama is idylique - (think Heidi-style green hills running into snow-capped mountains)

Saillon

Never been, it's in the valley beneath Ovronnaz. Doesn't appear to be time-limited, and they let kids in (under fours for free). Looks like there are more 'fun' things for kids than at Ovronnaz (water slide, and paddling pool in the summer)

Yverdon

This is the big, old one (Yverdon's full name is Yverdon-les-Bains). No under-threes.

And something completely different... If you happen to visit Geneva, then you could try going to "Bains des Pâquis". Located in the very city centre, on the Geneva lake, this place allows you to relax in sauna/hammam, swim in the lake and enjoy delicious fondue afterwards.

Booking in advance is recommended. Also check the opening hours. This association closes relatively early.

Another nice place I visit is Leukerbad

There are mountain activities (e.g skiing during winter, hiking during summertime) and two good public bath:

• Burgerbad Therme
• Walliser Alpentherme

If you plan to stay more than one day, a pass can be a solution as it gives free entrance to both bath (unlimited time) and to the cableways (Torrent and Gemmi).

• Summer offer
• Winter offer

After it depends as well if you have small children, on the second bath, they only allow children from 8 years old and further but is more quiet and less crowded. We really enjoyed it with my wife: it is perfectly romantic as well :)

For people with a smaller child I recommend to go only to the Burgerbad Therme because they even accept babies in some parts.

There are also other bath in some hotels, but I never tested as it is often reserved for the customers only.