i've been to st. croix, in fact, my folks are going there this week. i don't know anything about hotel stays or anything like that, since we just stay with relatives down there. but airfare is really expensive, prepare to shell out 500+ bucks a person for a flight down there as my folks found out. and you will need a passport (which is stupid, it's a us territory, not a third world country like it's trying to operate as). everything is way expensive down there (all food, not just the restaurants).
you should check out the rainforest just north of fredircksted, point udall, and chirstiensted especially. also, snorkel on a few of the beaches, especially near the two towns. it's been a while since i was there last, so i don't know too much in specifics. plan to rent a car for at least a couple of days during your stay to see some of this stuff, but the public transportation system isn't too bad from my last recalling.