Hi Biskut,
Hope i'm able to help a bit here
personally, i went to Anantara 2 years back. Stayed 4N water villa and 1N beach villa.
Excluding air tickets, we spent less than S$6k for 2 in total.
This include the accomodation + meals + 1 day island hopping for 2 + 1 night fishing for 2 + 1 spa session for 2.
as for how to choose, it really depends on what you want and your budget. If you don't mind spluring, there're many resorts that will WOW you and let you have an enjoyable stay (e.g. W Resort, Conrad, One Resort, etc).
For mid range one, you might want to try resorts like Anatara, Mirihi, Kandooma.
you can try
http://www.mal-dives.com/
this website has a number of resorts and will give u a feel of how much they will cost. Wu did all the bookings with them.
If you want to check with local agency, can try Holiday Tours. Many of us booked with them as well
http://www.holidaytours-sin.com/
For activities, apart from the usual snorkelling, diving, you can go fishing, sunset cruise, dolphin cruise and have some good spa sessions.
Resorts also organise various activites/tours. So you can check with the resorts what ar the activities available.
Some resorts have water sports facilities FOC or at a fee. You can kayak, swim or just chill out at your nice villas with a good book/magazine.
The view alone is enough to let u stone for 1 whole day...haha