Airport to ferry R/T: Book online. I've used CARM (https://www.cancunandrivieramaya.com/transfers/isla-mujeres.asp
) several times and they are very reliable.
Purchase your ferry tickets when you get to the ferry port.
Golf Cart Rental: You'll want one golf cart for your entire stay, and then the extra people can follow in a taxi. I haven't rented a golf cart in 2 years, so compare the companies. Most of the companies are on the same street as the ferry as only about a 2 minutes' walk. We would send 2 people to get the golf cart and the
rest of our group would take the luggage and head to the casa in taxis.
Golf Cart Rental cost: You may want to email the golf cart companies and ask if you can get a discount for paying in cash. I recall that I got a good discount for cash payment upon arrival.
Taxis: They run nonstop around the island. It's easy to flag one down. Make sure you have pesos to pay and ask the price before you go. I think I paid around 60 pesos to go from the ferry to the mid-island area near Mangoes restaurant.
Restaurants: There are so many good restaurants! Mid-island, I like Caribbean Brisas, and then Mangoes.