I had some success riding in Puerto Rico about 5 years ago.

I'd be curious about that. All I've seen of San Juan is lots of traffic.
The last time I was there on business - about a year and a half ago - a lot
of coastal buildings, including many businesses, had been flooded by a
hurricane and were vacant. That particular stretch of road was a little
quieter than the rest, but I still didn't see any areas that looked great
for cycling.

I should note, too, that I'm not a cement pounder. When I ride on a
vacation, I like to use a mountain bike or a cruiser and do about 10-12mph.
Perhaps it would help to know what intensity of riding Phil is looking for.