The tropical rain forest is the largest one in the Carribean. You have plenty of time to see someof the most amazing flora and fauna the island has to offer.
I have done the Bacardi tour and while it helps kill some time, it was a disappointment. You don't get to tour the actual plant, but a struture that was built to accomodate tourists. You do get two free little drinks and an insight into the Bacardi family and the rum making process.
Old San Juan is indeed worthwhile seeing. The old fort itself will take an hour or two to tour and there are some other great architectual structures to visit.
You may have some trouble finding shops selling jewelry and t-shirts,however,