Well, defintily Toby´s tips are all good!!
I´m from the south of Brasil, and I go often to Cumbuco (been there last week, 15knots morning and late afternoon, 25 knots between 2pm-5pm every day!!!)...
Well, I have a slightly different point of view...
I do not suggest you to rent a regular car in Cumbuco... the rental companys over there are not exactly professional. They may charge extra fees at the delivery moment, pointing at "sand in the engine", or some invisible scrap at the roof, or something else like that, and the cars are usually very well used...
Suggestions: rent a buggy at cumbuco, OR, pick a local buggy cab OR rent a car at the airport in fortaleza and bring/rent a GPS...
Rates at the airport are always higher, but the cars are all brand new, and they won´t give you any surprises fees at the return - and they accept credit cards.... "Localiza" or "Hertz" was always OK for me...!
and, of course... Welcome to Brazil!!!