I'm gonna be straight to CAS, checking out all the new things while creating a new sim. Then I'm going to check out the world, find a houseboat to live in, customize it and set sail! I will probably try to discover all uncharted islands and such before playing with resorts and mermaids :)
I definitely think that it's a rabbit hole that will work exactly like the mausoleum, and your sim will have crazy encounters in there that you can only read about. You will probably get attacked by the kraken there or some sea monster.
TS1: Loved them all! Makin' Magic was probably my favourite though! TS2: University (only because it took so long and got quite boring) TS3: WA, I like it but it is quite boring after a while and I don't usually send my sims on holiday/adventures. :-)