Can't comment on Mauritius (except wondering if you can get a cash + points stay at the St. Regis???), but can highly recommend the Andaman in Malaysia, which we visited a few months ago. Situated on a beautiful bay and on the edge of a jungle, with monkeys and other wildlife to be seen simply around the grounds or via complimentary or inexpensive nature tours. It was a unique experience. The island (Langkawi) that the Andaman on is a pretty relaxed place that is worth a day to bop around. I did a long write-up on the Andaman thread here in the Starwood forum, and there are many other useful reviews there as well.

Maldives, French Polynesia and Seychelles also fascinating options, though New Years is the rainier time of year in the latter two. Also, we stayed at some (very nice!) Hilton properties in those places, so afraid I can't comment on how good the Starwoods are there. (Not to sound like a Hilton advertisement, since generally I like Starwood better...but if you have oodles of Hilton points, pm me if you want further information about great Hiltons at those places or at Koh Samui.) In addition to FT, Tripadvisor is a good source for reviews and comparative rankings, though the FT crowd tends to be savvier.

Wherever you go, beware of mandatory, expensive New Years Eve dinners that some resorts have.

Good luck!