Hi Reppalc -
My colleague on thsi forum alerted me to your post. I'd like to help with some information.
However, I'm on the road this weekend, and don't have quick access to some of my material about St.
Gallen. I'll try to get backto you before September. ;-)
It is not overloaded with tourist in the way that Luzern is, but the old town and Abbey have a steady flow of tourists. I've not found them to be overwhelming in my visits there.
Briefly, the Abbey and Abbey Library, as a Unesco World Heritage site, are very impressive and worth investing several hours in the tour ( required) and exploration of the Abbey and surroundings.
Close by in the old town, which deserves a leisurely stroll, is a small 700 year old very Swiss restaurant.
I periodically ride the train over when I am staying in Zürich, maybe on a jet lag day, just to have lunch there.
There is nice hiking in the hills near Appenzell and Gais, as mentioned by Lucas.
Nere Ebenalp and the Seealpsee there are some nice walks that are not too mountainous.
More on that when I get back home.
There is a famous small mountain hotel/restaurant hanging on the side of the mountain near there. Name escapes me at the moment. . You have to climb up, which would test your "vibrance." ;-)
Switzerland is a hikers paradise...for all skill levels, and September is a great time to go:
There are sevral excellent resources for more details, if you are interestd.
To help me adjust my comments a bit-
where are you from, and do you speak or read any German, French, Italian?