Mostly I just wanted an excuse to post this composite photograph. Betcha can't spot where I spliced 'em together!
The Carbide and Carbon Building went up in 1929 to the designs of Daniel and Hubert Burnham (sons of the famous city planner). It represents the absolute pinnacle of Jazz Age elegance. Its materials are sumptuously rich, its ornament lavish, its design stylishly geometric, its massing in harmony with the spirit of the age (not to mention the zoning codes.)
If you've never been inside, do so. The public spaces aren't huge, but they are as stylish as the outside.
The building was originally office space and display areas for Union Carbide. It was converted to a Hard Rock Hotel in 2004, meaning the lobbies are essentially public space. Feel free to wander in! There are no pubic spaces on the upper floors, sadly, though one group of upper-floor suites does have an entry foyer dressed up as a sort of medieval monastery.
But it's no substitute for an observation deck. Imagine what it must be like to go up in the lantern!