Welcome to My Swiss Alps.
I like to travel in Switzerland in September. On average, it has the best weather patterns, although there are no promises in the Alps....the mountains make their own weather. ;-)
With a short trip such as yours, I'd skip Zürich and go directly to Luzern or Interlaken. Assuming that you travel by train, coming from Paris you can transfer at Basel for Luzern, for example.
Zürich is my favorite city in Switzerland, but it is best for the cultural items, and is not rewarding for a quick visit as Luzern. Zürich is not in the Alps; Luzern has striking mountains and a beautiful setting on the lake.
Either Luzern and surroundings or the area near the Jungdrau/Interlaken can be rewarding for weeks, so there is more than enough o do and see for a few days.
Luzern and Interlaken offer similar convenience to Milan. Luzern is close to the Gotthard pass route; Interlaken is closest to the Simplon route via Spiez/Brig/ Domodosola.
The time table and a map wil help you visualize that:
be sure to read the introductory instructions at that link.
This map is very helpful:
That map can be set to show train stations (etc.) by selecting Points of Interest/Traffic in the menus.
A rail pass is usually a good idea. These related links can get you started.
Arno and Annika can help you with the selection.
Hope that helps you get started.