Old San Juan - great city, great food - if you want to do some serious shopping go to Plaza las Americas, it's about 30 minutes from the port - if you like to rum, buy Don Q - you will not find it no where else. They have different flavors.
Good Shopping, but don't waste your entire day shopping. Go to the beach, I recommend Megans Bay and Coki Beach. If you want to take a taxi ask for Martha she is great. She is from Dominican Republic.
You can buy the English Harbor Rum here. I believe the store name is IHS (Celebrity recommend this store)
The price is good. You would not find it any where else. This rum is from Anguilla. We had a hard time trying to find it in the other island. St. Maarten had it but not the aged one.
365 beaches - we went to Church Beach, very nice beach. They have a restaurant at the beach. You must try Rum Punch - make sure you ask them to use English Harbor Rum, very good. I'm don't like much rum, but this rum is very smooth.
My favorite port - love St. Maarten (dutch side) and St. Martin (french side)
St. Maarten - if you are looking to liquor this is the island to buy it - it's cheaper in St. Maarten (dutch side)
Shopping is great.
St. Martin - you can take the public transportation to go to St. Martin - it cost $2 and is reliable. We ate at a bakery (can't remember the name but the name starts with an C) it's located where the flee market is at. They have great pastries.