Seems quite a few ships come into SanJuan late in the afternoon. We did this on Destiny as well. The ship does offer tours, but Old SanJuan is so close, I don't think I'd bother with any tours there.
U can walk the few blocks to Old SanJuan, and stroll around, and you'll see that's where many of the ships tour buses are.
If u want to see a bit of the night life, grab a taxi and head over to the El SanJuan Hotel. The disco and lobby bar there are hopping!