Given a near average delay of an hour and half to the LSL's arrival, you will have about 8 hours to wander and still be back without cutting it close. My favorite first thing to do on a Chicago layover is get a Chicago Dog at a cart on the Loop, but there may not be any on a day as traditionally deserted as Christmas. My favorite second thing to do is definitely an option: ride the L! It really is quite the railroad. If the weather is tolerable, I also recommend simply hanging out on the Clark/Lake or Adams/Wabash platform for a bit, facing the adjacent junction and watching the elevated do its thing. Red Line TO HOWARD (things get dodgy quickly south of downtown) and Brown Line are my rides of choice. If you're a fan of watching planes come and go, the Orange Line goes to Midway. Should trains with electric locomotives appeal, Metra's Electric Division terminates at Millenium Station, as does one of the US's last interurbans, the South Shore Line. With a layover of your length, I would normally recommend a visit to one of Chicagoland's "hot spots" - http://trn.trains.com/photos-videos/hotspots/2006/07/chicagos-hot-spots - but traffic on Christmas Day will be a fraction of its normal self.