South Beach is a good place for night life, walking around, people watching, the beach.
Seminole Hard Rock Casino for gambling, some shows. You will just miss Championship Boxing and Wayne Newton (maybe that is a good thing)
I would not miss Little Havana around 8th Street for great Cuban food. You missed the Calle Ocho Festival...
maybe Miami-Dade Metro Zoo
diving/snorkling
fly over to Bimini for a day
maybe charter boat for a half or full day for deep sea fishing
might be something going on at the convention center