From what I've been reading Old San Juan has a special police force. The police all speak English and there are a lot of them. This does not mean Old San Juan is not safe...it means it is safe and kept safe. If it wasn't they would loose the cruise business. ShoreAdventures.com is also a great site for finding things to do,
If you get off the ship as soon as it docks, you might want to try what we did. Took a cab up to the top of the hill, then walked down. I would not walk around any city at night, especially with kids, but you will have a couple of hours and it can give you a sense on the place. We found the people very friendly. We stopped at one point to consult a map we had and a young lady, a local, stopped to ask if we needed help. In some ports, that would make me leery but she really did care about helping us and told us we needed to go one block over, which turned out to be correct. Keep in mind though, that it is a very steep hill and rough pavements.
Hope you find something fun to do there. Wish I could go back...soon.
As we did not want to book an excursion and just explore on our own, we took a taxi and the driver took us to a great local beach...well not US standards...but it was just really beautiful..we went before sun went down...nice place indeed...I am looking to find how it was called...