Free entry to museums every now and then is a nice thing. Our entry fee helps to sustain their activities, but who says "no" to a thank you freebie every once in a while?
Milan's Archaeological Museum is set up on an area that used to be part of the city's ancient imperial Roman walls and circus...no kidding...that later housed the city's most prestigious female monastery, eventually overseen by none other than a Sforza abbess.
Have you been, but a long time ago? Go, again. They've renewed the exhibits quite a bit over the last few years.
Archaeological Museum of Milan
Corso Magenta, 15
http://www.comune.milano.it/wps/portal/ist/it/vivicitta/luoghicultura/Musei_Spazi_espositivi/Museo_Archeologico (in Italian)
Tues-Sun, 9:00 A.M.-5:30 P.M. (last entry, 5:00 P.M.)