If you like architecture, I agree South Beach is the way to go. Coral Gables also has some nice big houses, with big ole live oaks to shade the way. But you’re not going to get much nice architecture in south Florida. Hurricanes do a number there.
If you’re into nature, there’s the Florida Everglades. You can take an airboat tour and see a bunch of alligators, who are have been “trained” to come up to the boat and behave in unnatural ways. Or you can schlep up US 41 and go to the Shark River Slough park entrance, where you can take a tram ride and see the fabulousness that is the Everglades, in all of its natural splendor.
Not too far away is Butterfly World. Never been there, but people say its very cool. In Miami, there’s also Fairchild Botanical Gardens. It’s a nice one, though a bit pricey.
That’s all I got.