When in Zurich it is a must to go to the lake even on it if you have the luck to be there when it's a nice summer day.
Zurich is a rather small city compared to the biggest cities of other countries which is part of the reason that at one point it had the nickname downtown switzerland. Being small though is part of its charm and it has the benefit that you can go a long way by foot and seeing lots of things as everything is rather close together.
My favorite place is walking along the lake where you can see people enjoying the sun, talking, listening to music, sometimes a crazy person dancing through the crowd, ducks, swans, … There’s always something worth seeing near the lake when it’s sunny and the people are drawn outside… sometimes even when the clouds are hanging deep and it may rain soon though there will definitely be less people!
What I recently did was take the boat to go a bit further than Zurich and see a bit more of the lake. A boat trip invites you to do double exposures portraits with the agitated water:
or take pictures of the people on the boat enjoying the sun and the surroundings:
And if you do not want to go on a big boat and go as far as I did, you can always rent a pedalo:
Also there are a lot of events close by the lake the biggest one being the Zürifäscht taking place only every fourth year. The highlight being big fireworks accompanied by music. But also on New Year’s and on Switzerland’s national day August 1, there’s a big firework on the lake worth seeing: