I consider e/c natural wine shop a hidden treasure in Milan. Inside a building with an "Emergency" (that is actually an ONG) plaque outside, the first thing one might think is that is a hospital. Once passing the wall, there's a cute internal square, with Basilica di Sant'Eustorgio view. You can sit both inside or outside, which a strongly recommend for spring and summer days. A nice place for just passing by to grab a bottle of wine and drink it at home, having a glass of wine (they have many options) or enjoying a relaxed evening.

Best designer clothing store in Milan (in my humble opinion!).

Achille Castiglioni (1918-2002), architect and furniture designer, left an enormous amount of inventions and designs, carefully kept by his family in his studio museum in Piazza Castello.  Castiglioni's studio museum can be visited on appointment throughout the year.

Vintage interiors store selling mid-century and post-modern homeware.

Flowers lovers! Orto Botanico di Brera is an hidden treasure right in the center of Brera and near by Pinacoteca. The garden is a university and museum, there they organize events and activities for the public and schools. It has an amazing collection of flowers and medicinal plants, plus little corners where you can stop by and enjoy the silence for a bit.