Zürich offers plenty of very nice opportunities for jump into the cold on a nice summer day. Most of them are of course at the border of lake Zürich. If you like swimming in the cool river,  badi unterer letten is the place. It's a really authentic river bath. You don't need to be a very experienced river swimmer. You can just walk up the river and get out again at the big grid at the end of the bath. There's also a swimming pool and a nice pool for kids. If you want to meet zürichs creative people letten is the place.
A truly urban experience.

Flussbad Unterer Letten
Wasserwerkstrasse 141?8037 Zürich
Telefon 044 362 10 80

The "josefwiese" is one of the places to hang out on a sunny day. It's a good place to go with kids, next to the big lawn there is a playground and something between a pool and a big fountain.
There is always a bunch of people playing boule and there is even a beach volley field.
If you get hungry there's a little coffee place where you get good coffee and lovely tarts.

Relax here after an extended shopping tour through the nice shops of "Kreis 5" ( thats how this neighborhood is called).
It's also  close to the Löwenbräuareal with all it's art galleries.

View Zürich in a larger map