Three days is the first Miami trip where the question stops being what to leave out and becomes what to add. The two city days are already settled and they settle themselves: one on the mainland, one on the barrier island, one bay crossing between them. That is not an opinion, it is the geography. Miami and Miami Beach are separate cities across Biscayne Bay, joined only by causeways, and the six road miles between Wynwood and Ocean Drive cost fifteen minutes at ten in the morning or fifty at six in the evening.
So this page is about the third day, which is the only genuinely open decision in a three-day trip, and the one people get wrong by treating it as a menu rather than as arithmetic. Day three almost always means leaving the city, and leaving the city here is expensive in hours in a way that is not obvious from a map. Miami sits between two national parks and a hundred and sixty miles of highway islands, which sounds like abundance and is actually a trap: the drive west to the sawgrass is forty-five minutes to an hour, the drive south to Homestead is about an hour, and Key West is three and a half hours each way before you have done anything at all.
The table below prices each candidate day three in the only currency that matters, which is door-to-door hours from a city base, along with the departure window you actually have to hit and the time you realistically get back. Four of those options then get a full schedule: west to the airboat belt and the sawgrass, south to Homestead and the water that is a national park, north to Fort Lauderdale by train without touching a car, and stay, which is the option nobody writes up and which is the right answer more often than the others.
Two facts to carry into that decision, because both are widely got wrong. First, airboats do not run inside Everglades National Park. The airboat trips sold from Miami hotels operate on private land and in Miccosukee country along US-41. They are a genuinely good half day. They are not the national park, and if the park itself is the point, that is a different booking and a different clock. Second, Biscayne National Park is about 95 per cent water, so seeing it means getting on a boat out of Convoy Point near Homestead, which is an hour's drive before you board.
The last section is a short list of rules for day three: what not to book on that evening, when to hire the car and when to leave it, and the two months of the year when a day out west is the best day of the trip rather than the hottest.