As noted in
#3 the boat ride is worth it.
A 50 minute RT to Mt. Uteliberg is also nice.
Clearing security at ZRH (even in the morning) is pretty fast.
So once you get to the main station, walking along Bahnhofstrasse (lots of upscale shops to browse) to the end at Berkliplatz for the boat trip (at XX:10 and XX:40, in the summer) for most of the day might be nice. (15 to 20 minute walk).
[Inside Hauptbahnhof at the end of the great hall is the tourist info office...they could make further suggestions.'