In that case (that you are staying North), check out Rojo Lounge (Mata Grande area) - lots of fun, cool and hip.
On Sundays Rendevous (sp?) has an afternoon beach party with a chicken drop.
Fido's is in San Pedro town and has a great new band - Fridays are usually pretty happening there....
I'm sure others can help you with additional points of interest as there are fun places all over the island..