Whoever told you borading wouldn't begin until 7
m is misinformed. On these sailing that leave San Juan at night (in this case 10:00pm) you can actually begin boarding around 1:00pm and sometimes a little earlier. Go straight to the ship and check your bags, if you're hungry go onboard. They'll have the welcome aboard buffet set up. Then head off to see San Juan and come back to the ship. Don't be too late getting back, on the Adventure of the Seas last month they recommended 5:30pm even though the ship didn't sail until 10:00 pm also. My ship left PanAmerica pier but I believe Serenade goes out from Old San Juan pier (check to be sure). Old San Juan is right where you'll want to be, it's where most of the things to see are. The Bacardi factory, the old fortress, cobblestone streets. PanAmerica is a long way and inconvenient to shopping and sightseeing.
I didn't do anything anyway this time. Had a very early flight and needed a nap by the time I got there. Just blew off San Juan and figured I'll do it next time. I love the southern Carib so there will be a next time sometime soon.
Have a great cruise and let us know about it when you get back.