If you are in South Beach, you really should take a walk on famed Ocean Drive. The people watching is fun, the unique Art Deco style hotels are colorful, and you can recognize the scenes from many movies and TV shows dating back to Miami Vice and Scarface.
There are many restaurants and bars along Ocean Drive. Some are great and some are just tourist traps. Check the Trip Advisor reviews before you get hooked by someone promising a "special" deal. Many times it comes with strings or you pay more for drinks, etc. which negates what ever special was promised.
Some places I like: News Cafe, Wet Willies, Caffe Milano, Pelican, Fish Called Avalon.