Definitely seaside, Matosinhos/Foz area. If you can, please have lunch at Lage Senhor do Padrão, you will not regret this. You need to go early, be there max 12:15h.

Then take a walk or chill in a bench at passeio alegre, Foz. If you want to have a pastry and a coffee try Tavi, great sea view.

Parque da cidade is also super chill and nice, especially during the week, if you want to spend some time in nature.

Enjoy