Anywhere along Michigan Avenue - Sears, Bloomingdales, etc.
Avoid 'Victoria's Secret' - highly over-rated. When shopping for something for Mrs Shillard, I found Nordstrom to offer the best selection - and the most helpful staff. The attractive young female salesdroid recommended a new range of hipster 'thongs' because:
"They're really cute, and really comfortable - see?"
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Sears Tower (the building is no longer used as Sears HQ, that is now in a large campus at Hoffman Estates about 20 miles west of ORD) is down town on the lake front. Near-by is Navy Pier and Grant Park(short walk to the north), near the South Loop of The El (Elevated Train system). Depending on the weather, that area can be nice to wander around, browsing the boardwalk and boutiques at Navy Pier etc. Lots of good restaurants etc.
If you want the tall building experience, I'd go for the John Hancock Observatory rather than Sears Tower. I think the view is better there and the Magnificent Mile (Michigan Ave shopping district) is a short walk away. If you have time, the Chicago Architecture Foundation has river cruises with commentary on Chicago's landmark buildings.