A few ideas:
1. Bikes for "rent" (actually free). Bike halfway around the lake, and take the Ferry across and return. https://www.zuerich.com/en/visit/sport/zurich-on-wheels and Herzlich Willkommen - Zürichsee-Fähre Horgen-Meilen AG : Zürichsee-Fähre Horgen-Meilen AG
2. Take the tram up to the Uetliberg and go for a hike - up to the top/tower, then along the ridge. Take the cable car down and return. https://www.myswitzerland.com/en-ch/...felsenegg.html
3. National Museum near the main railway station is great.
4. Zoo is great if you like zoos.

Get a local transport pass: https://www.zvv.ch/zvv/en/travelcard...s.tab-tab.html and/or train ticket. There may be a pass that combines ZRH-city center train plus local transit; ask at the ticket counter in the ZRH rail station.