If you're looking for the historic charm of Milan, Vicolo dei Lavandai is one of the most charming places in the city centre. Wouldn't say it's a hidden gem, as you can easily spot it by walking along Naviglio Grande.
The name comes from what the alleway was used for in the past, to wash clothes.
Here there is also a shop where you can discover pictures of Milan in the past, a restaurant called El Brellin, and a cocktail bar called Morgante.