You may go to the Greifensee. There is a path around it (around 15 km), and it's super flat. Most of it (90%) is fairly compact gravel, the rest is tarmac. A part is currently under water (around 30 cm deep), but it can be by passed.
If you go by car, the best thing to do is go to Maur, and to park at the church, since the parking lot close to the lake is quite full.

Hope this helps

vei