Ahh, girls' only vacation! We did that a few years back and it was TERRIFIC- have fun!
Excursions- St. Thomas has mawvelous shopping, especially jewelry. Great prices everywhere. As for beaches, Megan's Bay is much better from far away than up close. We have been there 3 times and it gets stinkier and worse each time- some kind of sewage problem. Trunk Bay in St. John's was gorgeous, although it took quite a few dollars and a good bit of time to get there, but it was certainly worth it. Coki Beach was great but smaller than some others.
Puerto Rico has always been a disappointment for us. The most fun we had was going to Roberto Clemente's developmental baseball complex to watch some baseball games and buying commemorative baseball mitts. The beaches we tried were not very good at all. Old San Juan is nice if you want to shop and walk around. We will likely try the rain forest this trip. Hope its better than the other times we have been there.
Good luck in finding your favorite memories to make!