I'm sitting right now in Hotel Número Uno (really a large guest house) in lovely Ocean Park, San Juan. It's a beautiful property -- not in the Ritz-Carlton sense, but in terms of cleanliness and "leave-me-aloneness" it's perfect. I booked here on the recommendation of the NY Times and it has met their review perfectly.
The guest house is at the dead end of calle Santa Ana in a gated, guarded neighborhood and sits directly on the beach. When you wake up and fall asleep, all you hear are waves. The rooms are very simple, but very nicely appointed and modern (tiled floors, beatiful wood doors, strong A/C w/remote, high-speed internet, designer bathrooms).
There is dedicated staff to arrange your beach chairs, towels, umbrellas, etc. just where you want them between the guest house and the ocean. The in-house restaurant, Pamela's, is reported to be one of the best on the island. I have eaten there -- your choice of indoors or at tables right on the sand -- and the food and service really are superb.
The only drawbacks are that on some days (weekends, holidays) the beach can get quite busy in the late afternoon, but still mornings and nights are dreamy. The highrises of Condado and Isla Verde are in the distance east and west. The water has been ideal for swimming.
This isn't everyone's idea of an idyllic Caribbean vacation, but on the other hand it has a nice, exclusive feel and I can't believe I was here 15 minutes after landing in San Juan!