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There aren't too many opportunities to observe Miami from the top of a tall building from a "normal" observation deck. In fact, I am pretty sure none currently exist. Given the prevailing weather, rooftop bars are much more common.
Some of the highest of these are...
FIFTY Nightclub. This rooftop bar, on top of the Viceroy Hotel & Spa in Brickell, is a nightclub to be seen at. It does qualify by having 360 degrees of windows which you can look out, if you can find your way through the crowds, and if you can get in! The nightclub is open Friday and Saturday nights from 11pm to 5am.
The Bar at Level 25 is quite a bit less exclusive, if you find that you can't get into FIFTY, or find staying out past midnight to be a problem. It's on top of the Conrad Hotel and opens at 11:30 am.
Interested in more of a restaurant? Visit Area 31 on the 16th floor of the Epic Hotel downtown. It's open for breakfast at 7 am so you can catch the sunrise over Biscayne Bay and Miami Beach.
However, some other opportunities may present themselves in the future: