You can go anywhere in casual garb - but don't. Miami is tropical, but professional. Heavy starch induces heat strokes. Even taxis are inconvenient. The best way to get around is to rent a car.
Rental rates are low, and most companies offer cell-phone rentals for just a few dollars a day.
One reliable company is Avis, with offices throughout the city. Most agencies have fully bilingual clerical help, but specify if it's required. There are many other service centers listed in the Yellow Pages of the local telephone directory. Forget downtown, except for cheap electronics. Instead, head to Bal Harbour, one of Miami's ritziest suburbs.