A bike day
A fun way to visit the city attractions is by bicycle and we loved the fact that Zürich is so progressive as to offer free access to bicycles for anyone willing to ride them (a deposit of 20 francs and a photo ID are required). Zürirollt is what you should look for, they have many bike stations (Bellevue, Hauptbahnhof and Enge are near our suggested hotels) which are set up in old ship containers. You just show up, grab a bike and return it by the scheduled time (it varies depending on season). We loved riding along the lakeshore, the University Botanical gardens and its glass domes, the parks as well as a bit of the old town where we stopped for meals. It will be a particularly great experience on nice weather days!