I don't think I know a more relaxed city than amsterdam. The small alleys , the grachten , the house boats, people sitting outside and flowers everywhere. So maybe one does not need to go to a park to relax. Still a recommend a walk through Westpark. It was saturday when we were there and everywhere in the park people were having parties. Every few meter was a small gathering of people. Some were having a barbecue, others were having a birthday party with balloons and some came just for a picnic. A very special atmosphere I have to say.
I recommend walking to the west part of the park, after the main park you come into sort of a brown field - very beautiful.
There you also find the Cuulturpark Westergasfabriek and several restaurants and bars.

Air ‘kunstENhuis’ is located in a former brown field, nowadays transformed to a vibrant cultural park, named ‘Cultuurpark Westergasfabriek’, close to the city centre.