lucky you....Chicago is a great city.....
I've been to the exact site of Mrs. O'Leary's barn (now a CFD training facility!), the old Union Stockyards site (look for the memorial to 21 firefighters killed early in the 20th century - worst single FD accident until September 11), and the site of the famous 1893 World's Fair (Jackson Park - you'd never believe anything was ever there before)......I like offbeat historic spots like these......
downtown: walk thru the old Marshall Field's, of course, and get some Frango's mints.....take an architecture walking tour AND the river architecture boat trip both from the Chicago Architecture Foundation......look for the restored "L" stations.....the Navy Pier was NOT very interesting to me (more like a family hangout place).....
see the very spot John Dillinger was gunned down.....look for the original Playboy Mansion......visit the Jane Addams-Hull House, formerly in a notorious slum, now surrounded by UI Chicago and a Daley-era expressway
admittedly, my interests and yours may not match!.....rental bike kiosks are scattered downtown.....
a ton of stuff to see......