Best places to visit in Puerto Rico?

My friends and I are going to Puerto Rico tonight. Our tentative plan:

First night in San Sebastian to go to the Mask Festival in Hatillo.
Second night—???
Third Night – Fajardo
Fourth and Fifth nights – Vieques
Last day – visit Old San Juan and leave…

So, does anyone have suggestions for night two, and then fun stuff to do during the day?

Thanks!