Lists of free things in Miami are easy to write and hard to use, because the expensive part of a free day here is never the attraction. It is the distance between two of them.
Start from the transport map, because that is what decides whether a zero-dollar day is possible at all. On the mainland, the Metromover is fare-free: three loops, 21 stations, no ticket and no tap, covering downtown, the Omni and Brickell. On the barrier island, Miami Beach runs five free trolley routes, published as 8am to 11pm daily at roughly fifteen to twenty minute headways, from South Pointe Park at the southern tip to 88th Street in North Beach. Both are genuinely good. Neither touches the other. No free service crosses Biscayne Bay, because no rail line does and the trolleys stop at the city limit, which means the moment a free day involves both cities it stops being free and starts costing $20 to $40 in cars.
So the first decision on this page is not what to see. It is which side of the bay you are spending the day on, and the answer is whichever one you woke up on. Below are two complete days written to a clock, one for each side, and both of them cost nothing to move around.
The second thing to understand is that Miami's free layer is at its strongest on two specific dates each month, and they are worth planning a trip around rather than discovering by accident. On the second Saturday, the downtown art museum is free to everyone and, that same evening, the Wynwood Art Walk fills the arts district with galleries open late — with the ticketed mural courtyard historically opening free from about five to ten. On the last Friday, Viernes Culturales closes part of Calle Ocho between 13th and 22nd avenues from five to ten for a free arts and music festival, including a free walking tour led by a city historian. Those two dates are the difference between a free day that is adequate and a free day that is better than most paid ones.
The third is where free quietly stops being free, which in this city is almost always a vehicle. A car turns a free beach into $4 an hour, a free island into a toll plus a parking fee, and a free afternoon in Wynwood into a lot charge. The single most effective budget decision you can make here is to not drive, and the second is to know the handful of places where arriving on foot or by bicycle costs a fraction of arriving in a car.
Everything below is free unless it says otherwise, and where something is not free it says what it actually costs.