Thanks for all the wonderful advice and encouragement. We're going to investigate the ferry tomorrow and if it doesn't work out for any reason will go back to the airline option.
Not really considering other islands, the main reason is to spend a few days with friends who will be there - the only other island we were considering was Cuba where we also have friends, but we decided to leave that for a later date - maybe Semana Santa. Our
PR friends have already put together an itinerary, which sounds great!
Day 1: Tour of Old San Juan, surrounding beaches (Isla Verde, Pinones, Loiza) Condado for dinner/visit other friends
Day 2: Tour of inner island and South (lunch at Guavate, Cayey - lechonera and see live salsa dancing), maybe Ponce, hot springs at Coamo or Lake Guajataca on the way back
Days 3,4,5 - Vieques or Culebra (off Eastern tip of Island) to stay through New Year's.
Day 6 - Return to San Juan - stop by food kiosks and El Yunque Rain Forest on the way or visit Plaza Las Americas (plenty of options to chose from: Marshall's, Gap, Walmart, Office Max, Computer World, Macy's, Penny's, Sears, Borders, whatever...!). Old San Juan at night.