REAS Pkg is the Romantic Escape at Sea Package which is a special add-on on top of the cruise fare. It inlcudes a couples massage, dinner at Palo, a gift basket, special turndown service, and breakfast in bed. I think it's about $300 extra.
Palo is a alternative restaurant on the ship that serves gourmet Italian cuisine and has a extra charge versus dining in the main dining rooms. I think it's $10 per person but I could be wrong. It is only for guests 18 and over.
Castaway Club is a program for repeat guests. They're offered a special check-in line, a gift in the stateroom (a towel, beach bag, etc.), and a special reception on board. You are automatically enrolled after your first sailing.
Things that are not included in the cruise price/things you'll need to budget for:
Alcoholic beverages
Spa Services
Shore Excursions
Gratuities
Bingo
Photo taken by ship photographer
Tranfers to/from the airport to the port
Passports (needed for Caribbean travel after Dec 2005)
Nursery care for kids too young for the club (someone correct me if I'm wrong, but isn't there a charge for this?)
