Fortaleza's main attractions are the beaches outside of the city, including the Morro Branco with its multicolored cliffs, the glorious sand dunes of Canoa Quebrada, and the stunningly beautiful and rustic Jericoacoara. The city itself has a small historic center that's worth a visit if you have a day to spare, but it's certainly not worth the trip by itself. The best way to get out and see the beaches is to head out on a day trip to Cumbuco or Morro Branco; both are only a short distance from Fortaleza. These beaches are playgrounds for adults and children alike. Buggy tours, sand boarding, sand tobogganing, parasailing, boat rides -- you name it and you can experience it, all under a hot tropical sun.
Canoa Quebrada is also within easy day-trip distance, but you may want to stay a day or two to get a better taste of the place. There are plenty of pousadas and restaurants here, and a small but active nightlife scene.
To enjoy Jericoacoara you need at least 3 or 4 days; getting there takes almost a day. This isolated beach community features Sahara-like sand dunes, stunning white beaches, plus a steady wind that makes it Brazil's windsurf capital.