Giardini della Guastalla is the oldest park in Milan, dating back to 1555 when it was created by a young widow Countess Guastalla for destitute noble girls as a private sanctuary. It features a beautiful baroque fish pool alongside rare and well-tended flora, with statuary and neoclassical architecture that give it a peaceful atmosphere.
It's not on the touristy side of the city, so expect to find here many locals and students, especially if you visit during the week as it's super close to the Statale University.
I love to come here from time to time to relax, especially on a sunny day.