US Virgin Islands in December
 

Family of 4, 2 parents, 2 soon to be college grads looking for a graduation trip for all of us. Like snorkeling, 3 of 4 are certified divers, moderate hiking. Don't really like staying in hotels, prefer house rentals or similar. 10-14 days around Christmas.

and your question is?

That sounds great. St. John is probably going to be your destination, but it will be expensive both in terms of airfare and lodging costs, and there are likely to be minimum-stay requirements. However, since you want to spend up to 2 weeks that should alleviate the minimum-stay problems. There are a LOT of villa rentals (few condos). The island also has good diving, nearby as well as excellent snorkeling and hiking on the island.

what doug said! go to vrbo, homeaway or flipkey. or search st. john villa rental & you'll find a bunch of companies. almost no villas on the beach so you want a pool, super view & ac at least in the bedrooms. you will need a rental jeep. most houses offer housekeeping on some basis & some will do provisioning - shop for your groceries a/or booze.
you fly to st thomas & ferry over from charlotte amalie in 45 mins or drive 45 mins to red hook & ferry in 20 mins. if you're doing your own grocery shopping you may wish to shop on st thomas which has much bigger stores.
fyi, i'd book the air as soon as you can & your house not far behind. very busy time of year.

St. John has GREAT hiking. Like Virgina said, a rental car is a good idea and they often sell out in the high season. Book now if you haven't already! The Fish Bay area on St John has some of the nicest villas and views. There are 2 dive operators on island and at least one of them offer "introduction to Scuba" classes for uncertified divers. BRING PASSPORTS... The British Virgin Islands are close by and you'll want to visit them either by organized excursion or boat charter.