Pizzeria Uno (or Uno Chicago Grill) claims to have invented the deep dish pizza, so you could go to 29 E. Ohio St. to check it out. However, that is outside the Loop area. You can take the Red Line L train (along State Street) to the Grand Ave. stop to get there pretty easily though.
Keeping to the Loop area though (http://webapps.cityofchicago.org/LandmarksWeb/mapDetail.do?regID=6 ), here are what I consider must sees and eats:
* Willis (errr...Sears) Tower (http://www.willistower.com/ ). The tower has a new viewing area where you can step out and feel like you're walking on air. You can eat the Venice Cafe while there too (http://chicago.metromix.com/restaurants/italian/venice-cafe-loop/135723/content ).
* If you have any interest in a major American financial center, you can check out the CBOE and CME (global headquarters - http://www.cmegroup.com/company/visit/ ) area of the Financial District.
* Harold Washington Library Center (http://www.chipublib.org/branch/details/library/harold-washington/p/Tour01// ) - Seriously, this building is amazing - inside and out.
* Chicago Cultural Center (http://www.chicagoculturalcenter.org , on NW corner of Washington and Michigan) - If you want some background on all things Chicago and check out local creativity, this lovely building is cool to explore. Plus, it's free.
* Millennium Park (http://www.millenniumpark.org/ , between Randolph St. and Monroe Street on Michigan) - you could go ice skating. ;-)
* The Art Institute of Chicago (http://www.artic.edu/aic/ , Washington & Michigan - east side)
Go across the street to see all the art you can handle. They opened a fabulous Modern Art Wing this summer. If you don't want to go in (for some crazy reason), just take a picture with the lions out front and check out the other outdoor displays around the building. There are weekly free days too.
* Grant Park - Buckingham Fountain (although, it will be turned off, since it's winter). There are other statues and monuments along the way though, plus you can go through the park to get to Lake Michigan. See http://www.cpdit01.com/resources/buckingham_fountain.cfm and http://www.chicagoparkdistrict.com/index.cfm/fuseaction/parks.detail/object_id/83aa6305-adbe-4d8a-b333-004449057ea9.cfm .
* Just walk down State Street or Wabash if you want to shop.
* Printers Row (http://www.chicagotraveler.com/neighborhoods/printers-row-feature.htm , http://www.centerstagechicago.com/other/articles/L-harrison.html , and http://www.centerstagechicago.com/neighborhoods/printers-row.html )
Lunch in the Loop (http://www.lunchintheloop.com/new/ ) might help you find places to eat. Chicago has a lot of restaurants though. You've only provided a small clue as to what you might want to check out (i.e. pizza). Since nothing was stated about budget constraints or other cuisine preferences, I'll just mention some well known spots:
* Tavern at the Park (http://chicago.metromix.com/restaurants/contemporary/tavern-at-the-park-grant-park-museums/146477/content )
* Walnut Room (http://chicago.metromix.com/restaurants/traditional_classic/walnut-room-loop/137033/content , plus you can check out Macy's [errr... always will be Marshall Field's in my heart]). Also, Frontera Fresco (http://chicago.metromix.com/restaurants/mexican/frontera-fresco-loop/145159/content ) is in Macy's.
* Roof (http://chicago.metromix.com/bars-and-clubs/dj/roof-loop/877714/content ) - Eat and drink from the top of the loop in this rooftop bar at the new Wit Hotel (which is supposed to be witty). It's a pricey but interesting experience.
* Walking around can be exhausting. Did I mention that it's cold? You may want to get some coffee and relax at Intelligensia Coffee & Tea (http://chicago.metromix.com/restaurants/restaurant/intelligensia-coffee-and-tea-loop/145650/content ) or enjoy great tea at Argo Tea locations (basically, Randolph and State St. or Madison and Dearborn).
* Want to eat fast on the cheap? Don't hesitate to walk into places that are scattered around Chicago - like Jimmy John's Sandwiches, Potbelly Sandwich Works, Haifa Caffe, Sopraffina Market Caffe, Garrett Popcorn (not so fast sometimes, but good if you like greasy popcorn), Wow Bao, and Giordano's (decent pizza - thin or thick).
* Morton's (http://chicago.metromix.com/restaurants/steaks/mortons-the-steakhouse-loop/1179250/content )... Steak -- done well (from what I hear).
* Russian Tea Time (http://chicago.metromix.com/restaurants/russian/russian-tea-time-grant-park-museums/136416/content )
* Catch 35 (http://chicago.metromix.com/restaurants/seafood/catch-35-loop/136470/content ) - It's expensive but worth it for excellent seafood.
* Heaven on Seven (http://chicago.metromix.com/restaurants/cajun_creole/heaven-on-seven-loop/133537/content )
* Trattoria No. 10 (http://chicago.metromix.com/restaurants/italian/trattoria-no-10-loop/135481/content ) - fine dining, great for theater crowds
* Berghoff - (http://chicago.metromix.com/restaurants/austrian/berghoff-loop/146759/content ) - solid German food
* Zapatista (http://chicago.metromix.com/restaurants/mexican/zapatista-south-loop/145916/content ) - a great place for Mexican food in the South Loop
If you want authentic, heart-clogging Chicago food (polish, hot dogs, burgers, and etc.), then it's best to go outside the loop. Check out Jim's Original (http://www.jimsoriginal.com/jimsoriginal/Welcome.html ) or Express Grill (http://www.centerstagechicago.com/restaurants/expressgrill.html ), both open 24/7. However, in a pinch, anything from the Portillo's chain of restaurants will do (http://www.portillos.com ).
Beyond the above, search these sites for events or places:
http://chicago.metromix.com/
http://www.wttw.com/main.taf?p=1,5
http://www.centerstagechicago.com/
http://www.190north.com/
http://www.chicagoreader.com/chicago/Home
http://flavorpill.com/chicago
http://chicago.citysearch.com/