Where are you staying? What do you like to eat? What's your budget?
For starters everyone is going to recommend Chicago Style Pizza . If you are staying downtown, I would try
Pizano's or Giordano's. (IMHO Pizano's has a better crust, but Giordano's is good and there are more of them). Pizano's also makes a better thin crust pizza.
For quick lunches, if you are not afraid of Cholesterol, I would try a
Portillos. They are part of a chain now, but it is a Chicago original. The beef and sausage sandwich with both types of peppers is a go to. Also you can never go wrong with a Chicago Style Hot Dog.
If you like beer try
revolution brewing or
piece brewery. They are both brew pubs that are easy to get to off the blue line.
As far as other eating options, as OrangeFoo pointed out above there are so many great restaurants it's hard to make general recommendations without more information.
As far as things to do, again it all depends on what you are looking for, but as far as touristy things go, if the weather permits, I would try an architectural boat tour. They are great way to learn more about the city and get your bearings. I make everyone who comes to visit me go on at least one. Also if you get a gorgeous fall day try to make it to one of the lake front parks. They are absolutely amazing.