I live in the Loop (downtown Chicago). Definately can take the Blue Line ("L") downtown. If you're looking for great deep dish pizza, def check out Giordanos.

THe train ride takes 45 minutes from the loop to Clark & Lake (1st stop in the downtown area). Get off at Clark and Lake, walk up the stairs and on the street, walk in the direction the cars are going (East). Keep walking, you'll get to Michigan Avenue in a couple of blocks. Cross Michigan Avenue, and on the right side is a Giordanos. Get yourself a small deep dish, a beer, and enjoy. If you have a few minutes to spare, you could walk south along Michigan Avenue to Millennium Park, its only 2 blocks from there. Walk around, see the Cloud Gate, the fountain faces, and then once you're due back, walk west on Randolph or Washington. There's a blue line stop at Washington & Dearborne, and you can take this back to Ohare.

Enjoy!