Zurich is at its best in the summer, when swimming becomes part of the daily agenda. There are a variety of great places along the river Limmat, most of which are free and open to the public. When it gets too crowded at these, try to find one of the few hidden places along the lake.
The Hardbrücke is a long, brutal concrete bridge that runs through a former industrial area. I like the view down to the rail tracks and the many construction sites. And, within spitting distance, you can find a few good places to spend a nice evening, such as Helsinki (for concerts), Rosso (for pizza), and Les Halles (for beer and tapas).