We are in a very similar spot. We are planning to take the free airport shuttle to the Tri-Rail. Then we plan to take the Tri-Rail to the Miami airport. The train is less than $5 per person. We are staying at the Days Inn and it runs a free shuttle to and from the airport and to and from the cruise port. The only leg of the trip not covered is from the ship back to the hotel as they only offer rides one way. We may just uber for that. Im a little nervous about dragging all our luggage on the train but since it goes to the airport, hopefully there will be other people also lugging big bags around. What this plan lacks in cost it makes up for in time, risk of getting lost, and adventure, but we are flying in the day before the cruise and out the day after so we have time for a little adventure.