We did a week on Culebra a few years ago. Stayed in a rickety old building on Flamenco Beach. Beautiful beach, and other than the accomodations, we had fun. the most fun was being taken by boat out to a little island for the day (they dropped us off and picked us up). the snorkeling there was fantastic and the beach was beautiful. On the whole, enjoyable vaca. We were a bit out of the way, so we had to drive for all food, some good restaurants, small but tasty. Not a lot to do other than beaching. Sightseeing could all be done in a day. A really quaint little island with beautiful beaches. We flew directly out of San Juan on a Caravan to Culebra, and the scenery on that flight would rival the flight to SP. Landing on Culebra is a hoot, you come in between two large hills, drop quickly while makinga 40 degree turn to catch the landing strip.
We did end up staying 3 days in Old San Juan on the return. Toured the fort and generally just walked around. Great sights, great food. We previously had spent time in Rincon too, and we really enjoy PR.

Been there, done that, the washing machine ate the T-shirt