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. Swim along the "Riviera" - https://www.theguardian.com/travel/2...-zurich-geneva (and depending on exactly when perhaps a float down the Limmat? https://www.limmatschwimmen.ch/limmatschwimmen )
4. National Museum and Zoo are both great.