Typical Brazilian cars
Most companies demand that payment is made with a credit card. Also, most contracts state that the rental companies are allowed to transfer fines to the driver, and charge the respective credit card, for a period of up to thirty days after the car is returned; this happens because, when a transit infraction is commited, Brazilian legislation says that the owner of the car (the car rental) is charged; by signing the contract, you agree with the transferring of the charges to you.
Prices vary by much, starting at around R$ 50 for a basic car (weak engine, no air conditioner). Prices can go up quickly for more comfortable cars; a Honda Civic or a Toyota Corolla are considered top of line in Brazil. Insurance may or may not be included; an extra fee may apply to mileages above a limit; make sure you read all the fine prints of the contract, particularly in the case of 'promotional prices'. For a list of cars available and typical prices, check out this page.
Before renting a car, consider also that the beach area of Fortaleza is well served by urban buses, and taxis are relatively cheap.
Car rentals in Fortaleza
There are many car rentals in Fortaleza, from giant Avis to companies with a two or three-car fleet.
Most often, the staff won't speak any English. The companies below have websites in English.
Mare Rent a Car. Website in Portuguese, but they claim they speak English.
Costa Dourada Rent a Car.
Dunas Rent a Car.