Sunset from El Morro
For the best sunset in Old San Juan, get yourself to El Morro around 6:30 (or check times for sunset). El Morro is the large fort that is one of the prime tourist attractions in Old San Juan. Although the fort itself is closed at this time, you can head to the western most point of the fort, which is the most western point of Old San Juan. If you don't have time/energy to head to El Morro, you can see a pretty decent sunset from La Puerta de Old San Juan (Old San Juan Gate).
Sunset from El Morro
Sun setting behind El Morro
Sunset from street in Isla Verde
Isla Verde is a tough sunset to see because of the big buildings and the shape of the coastline. Your best bet is probably the beach, but make sure you plant yourself a little further east (Cafe La Plage is a nice spot, see post) because if you are too far west, the buildings will block your view.
Sunset from pedestrian bridge, Isla Verde
Sunset from Cafe La Plage, Beach in Isla Verde
Sunset from mountaintop, Culebra
I will write more about Culebra in another post but for now, I will tell you how to get the best sunset view. Now Culebra has no street names, so instead ask someone how to get to El Eden restaurant. There is a little dirt road that leads to the restaurant. At the entrance to the dirt road, and across from a little garden shop, you will see a somewhat wide dirt trail heading up the mountain. Walk up, bearing left when you come to a fork in the trail, until you reach a giant concrete structure. Climb up to the top (you will see railings and there are stairs around back) and that will be your best view, plus a pretty good view of the bays and Dewey, the only town in Culebra. The walk should only take you 10 minutes and is not intensive at all, just be careful walking downhill if you're in flip flops.
So there you have it, my recommendations for some of the best places in Puerto Rico to see the sunset. If you have any others, let me know and I will try to visit and post some pictures!
I've been out of town for the past few days, but I've got lots and lots to write about, including quite a bit on the island of Culebra, so stay tuned!