A large urban park located between Castle Sforza and Piazza Sempione, you'll find that if you want to meet new people in Milan, then this may be best place to brush up on your Italian and join in on a game of soccer or frisbee.
The grounds were originally intended as a military parade ground for Napoleon Bonaparte, but he fell from favor and the structures created in his honor (like the wonderful Arco della Pace and the Arena Civica) continued on as part of the public park. Other notable structures include the Torre Branca, a popular tower from which to get a panorama of the city, and the Pallazzo del Arte, a museum that focuses on Italian design from architecture to media arts.
If you're out to just relax in the sun or to soak in a bit of the Milan culture that exists beyond the fashion shows and shopping streets, then Sempione Park is a nice addition to a group itinerary.
Note:Castle Sforza (Castello Sforzesco in Italian) is an excellent cultural edifice for those so inclined; not only is it a large and attractive 15th century castle, but it houses a number of museums that include the Sforza Castle Pinacoteca (highlighting masterpieces of Italian painting) and the Museum of Ancient Art (a basis in sculpture), among others.