I'm looking for recommendations for a really nice day spa to send my mother for a day that's doable with public transport from Winterthur/Zurich as a day trip. Somewhere with nice views would be a bonus. I sent her to the Dolder last time she was over so somewhere else would be good.

How about this?
http://www.thermalbad-zuerich.ch/index.asp?kat=home

We went to Bad Ragaz (me + OH + her mum) on a day trip from Zurich. Lovely scenery on the train, nice easy day. It's about a 15-20m walk from the train station in Bad Ragaz to the spa.

The hotels alongside are magnificent, with magnificent prices to match! We contented ourselves with looking at the outsides of them

http://www.myswitzerland.com/en/bad-ragaz.html

Thanks! That's exactly the kind of place I was looking for.

A colleague told me about: http://www.saentispark.ch/

She said it was excellent.

Well the options suggested here are fine. However, if you want to try something unconventional, I would recommend the Therme in Vals, Graubünden.

http://www.therme-vals.ch/en/

It is not the closest place to ZH, but you can make it for a short week-end or even one day (but you would spend lot of time on the road). The true good thing about it is the design of the place and the location, in the heart of the mountains. Really wonderful.