You can leave your luggage at the left-luggage place landside in the terminal.

It's easy to get downtown via Metro (Orange Line, iirc), with restaurants and museums to entertain you for a significant part of the day, aided by the aforementioned free train loop.

I also recommend Vizcaya, accessible by Metro plus a <1 mile walk that also shows you the south end of I-95 from above.

Yes, dress for hot, humid weather and drink plenty of water.

Enjoy!