While in Miami you do not want to miss South Beach both during the day and at night.
Key Biscayne is also worth a visit where you can rent Jet Skis and cruise around Biscayne Bay.
Everglades National Park is unique and interesting, especially if you take an airboat ride.
As another poster said, going to the Keys can be a blast even if you do not make it all the way down to Key West.
Have fun and good luck.
