If you like to run I highly recommend running a lap around Årstaviken in Södermalm! On one side you have a forest, on the other side you have södermalm with little gardens, cafés etc, and the whole run is right by the water at all times. From Hornstull and over the bridge Liljeholmsbron and over to Skanstull and the bridge to the Södermalm side is the best part - that loop around is about 8 km.
There are a few outdoor gyms along this run too - which are free and can be used all year round. The one next to Eriksdalsbadet on the Södermalm side is the best and biggest, but the ones on the Årsta side are not bad either.
120 60 Stockholm, Sweden