A half-day in the South Beach area can be pleasant - park in the public garage at 13th Street and Collins, and you're just one short block from Ocean Drive and the sand. People-watch and/or eat, then head back to the mainland.
Not knowing your interests, it's hard to throw out suggestions, a couple might include the Merrick House in Coral Gables if you're interested in history (an easy drive from your hotel) - there's also a Miami Marlins baseball game at 07:10 pm in the new stadium.