Extra day in Milan

Pinacoteca di Brera (one of the most important art galleries in Europe). Castello Sforzesco (Sforza Castle). Several museums are inside. Galleria Vittorio Emanuele (the oldest modern Mall) Teatro alla Scala (Opera)
Via Montenapoleone (high fashion shopping) and at night some nightlife along the Navigli (canals) Not really hidden gems, but gems nevertheless. Go to Wikipedia and you'll find plenty of info on all of the above. Not sure you can do all of the above in a day, but the Pinacoteca di Brera and the Galleria should be a must.

Roberto gave most of the good suggestions I was going to give! Yes, the Brera neighborhood is also atmospheric and a good place for a meal after the gallery. La Scala is one of the best opera house in the world. Tickets can be pricey but you can also check out the line for discount last minute tickets at the side of the theater. If you just want to peek inside, there's a good museum and tour which also gives you a view of the inside of the opera house. I also liked the Risorgimento museum in Milan. http://www.museodelrisorgimento.mi.it/ Also right next to the Duomo is the Galleria Vittorio Emanuelle II -- one of the world's first enclosed shopping 'centers' and quite a jewel. http://en.wikipedia.org/wiki/Galleria_Vittorio_Emanuele_II

Thanks all - I was sort of "meh" about Milan - it was just a place to hang our hat before our trip home but now I am excited. Milan seems like the sort of place my husband likes very much - I like seedy atmospheric walled cities and he likes hustling bustling metropolitan centers.