Lakes and Damms
This lake is close to Jena, just a few km north of the city. So you can easily cycle there.
It belongs to the camping place and the entry to the lake is 3€.
There is a big beach where you can easily swim or take a dip as a non-swimmer.
Rent Pedal-boats and SUP
try nudism, it is allowed :D
How to get there by public transport:
The train is the best solution since there is a train stop called "Porstendorf"
The bus which is not that frequent.