What does a good day at the beach look like?

We will be visiting friends in June and will have a full day at Miami/South Beach.

We don't have an itinerary planned so that's why I'm here.

Note that I'm talking about family fun type things, it's me and my teenage kids.

We want to spend the morning walking around to see the best of Miami sights and eat some nice Cuban food for lunch. I just don't know where to go walking around.

Then we want to go swim at the beach, maybe South Beach since we will be closer there.

We are driving up from Homestead so probably going to stay around the southern part of the city.