MySwitzerland proposes 32 great hikes, and this one looks particularly beautiful: http://www.myswitzerland.com/en/interests/hiking1/hiking-suggestions/top-hikes/senda-ruinaulta-the-swiss-rhine-gorge-route.html. The legend goes that after rainy days, the fairies of Lake Cauma emerge to get some sun, so it might be a unique opportunity to see them...

I'll leave at 8:37 from Zurich with this train:

Zürich HB dép. 08:37 Gleis 7
Chur arr. 09:52
Chur dép. 09:58
Laax GR, Staderas arr. 10:32

and be back at around 4-5. If you're interested in joining, I'll be under the Blue Angel in Zurich HB at 8:30 (buy your ticket before).

Swimming in the lake seems to be an option, so bring along bathsuits, just in case. Bring along lunch (there are also restaurants along the way), etc.