A nice and refreshing walk in the middle of the nature can be done close to Castell de Montjuic. An easy walk with very nice views over the city and the sea.
Starting from the front entrance of the castle, turn left to reach the walk path that is close to the sea. This small path is long 1km (not even 1 mile) and ends in the Mirador del Migdia, an elevated place with views along the coast where the sunset is particularly beautiful.
On your way back, you can decide to return following the same path or taking other paths to perform a circular walk and enjoy other elements of the castle
This link here shows the details of this easy and nice walk in a sunny day: