MIA can be a good layover airport if you choose one target: a meal, a waterfront walk, or a short neighborhood visit. Beach plans need a bigger buffer than they look like they need.
What to do with bags
MIA lists a baggage checkroom/storage facility on the second level of Central Terminal E, open daily with daily rates. Confirm hours before relying on it for a late arrival.
Leaving the airport
Little Havana, Coral Gables, and close hotel/restaurant plans are more realistic than squeezing in South Beach unless the layover is long and traffic is friendly.
Getting back through security
Return with your onward boarding pass and acceptable ID. International connections may need extra customs, bag, or terminal time.