They usually close at 5 p.m. so I would not plan on shopping in SJ. Take a walking tour of Old San Juan or hop on the free trolley if it runs in the evening. Go all the way up to El Morro and walk down from there or walk along the Princessa to the fort. It is a beautiful walk.
A walking tour of Old San Juan is a great recommendation. We took a cab to the forts only because we weren't sure how to get up there. We walked around them watched the sunset and took lots of pictures. It is really something to see. Even just the houses and buildings between the forts were really cool looking. We could not tour inside the forts because they closed at 5pm. After we stopped at Amanda's Mexican Cafe (across the street from San Cristobol fort) and had an excellent dinner and margaritas, we walked back towards the ship. We found several souvenir shops open along the way. Typical Caribbean souvenirs. I bought a pareo/sarong for only $10. They cost $40 at home, so I was really happy. There's a casino right by the ships dock and we stopped there - no luck - but we had a great time.