In December, come at night for "ZooLights", when the place is lit up with literally millions of Christmas lights. You have to get tickets online, but on Mondays they are free. This makes a great family activity!
We came up on the the CTA bus #151 from our hotel near the Mag Mile, then had dinner at the wonderful "Mon Ami Gabi" French restaurant just a block from the zoo entrance before heading in. Folks who have visited Las Vegas might know the name of this restaurant. Turns out, the original is in Chicago, and it is even more fabulous!
The lights are magical! And the place is packed with locals who enjoy this winter institution. Allow about one to two hours for you tour of the lights within the zoo.
So if you are in Chicago in the Christmas season, head north to Lincoln Park and enjoy this wonderful local winter institution. If you have the time (and don't mind spending some $$$), enjoy a dinner at Mon Ami Gabi before or after your visit.