If she, too, has done a lot of travelling -- especially in Europe -- then I would second the suggestion for Lugano. It is a small Swiss city in the Italian canton of Ticino. If you fly into MXP you can be in Lugano in about an hour by bus or car.

If she has not done a lot of travelling, Lugano is out. It simply doesn't have the "Wow! I never thought I would actually see THAT in person!" factor. No famous sites, no spots that everybody should see at least once.

Take her to Paris or Venice. No woman has ever turned down a proposal made in a gondola floating down the Grand Canal (possibly because they are afraid they will be made to swim back to the hotel.)