If you go downtown, do yourself a favor and go check out the Mob Museum. But give yourself a few hours because there's a TON to go through. But it's very fascinating.
Afterward, go down the street to the Mob Bar. If you check in on Foursquare, you'll get a free appetizer (and if you get an appetizer, get the chicken parm sliders OMG).
Last time we were there, my wife and I sat at the bar, threw $20 each into the video poker machines, and ordered a couple of cocktails (which were fantastic) and apps. Because of the Foursquare checkin, we got one of the apps for free, and because we played video poker, she comped both of our cocktails. And then I won $60 playing poker. Not a bad trip.