Fajardo is a family town, I didn't see a lot of nightlife that didn't involve taking the kids (or your school chums) down to the park or the beach walk and socializing. But, I am not a 'clubber' and didn't seek out anything.
Rosevelt Roads, the largest Navy base in the world is about 10 minutes south of Fajardo, and where there are sailors, there are bound to be night life. I just didn't seek anything out.
Dress Code: Unless you are at the beach, you will see Puertoricenos wearing long pants and a button up shirt most of the time. Not fancy, sometimes well worn, but still not bumming around.
Puertoricenas (the female of the species) tends to dress to impress whenever possible, usually to focus attention on her physical 'assets'. If you want the attention, dress that way, because you will get it.