St Gallen could be also another place to consider. There is a direct train, that takes about 1h15min to get there. Just seeing the Abbey of St. Gallen, a UNESCO World Heritage Site, it is worth the trip. Its library contains books which date to the 9th century and is very beautiful. I also recommend to take a walk downtown center - the building and their balconies are just lovely.

Also, if you want a true Swiss experience, you can also visit Appenzell. It has also the advantage that most shops are open on sunday. Or, just go further down to Lindau (already in Germany), another lovely. Lindau was also preserved from the bombing during WWII, and has the oldest buildings in Germany. But, just to see the Bodensee, it is worhwhile to go there

Best of luck with your parents,

Lucy