lisaob...
we just made our first trip to AC and it sounds like you are looking for the same things we were...
we were up before the sun...watching it rise from our balcony with the first morning coffee...then after breakfast...hanging out on the beach for a bit...then a liesurly walk into town along the beach which is always uncrowded with great places to stop and rest, watch the birds, take pictures, etc.
we filled a few days with snorkeling out on the reef...and another couple by renting a golf cart and touring about the north and south ends of the island...and stopped at a few great restaurants/bars when the mood hit us. we were always back at our hotel fairly early and didn't check out much night life.
the people on AC are very friendly...from kids to old folks...everyone smiles and you will find it disarming and relaxing! no hurry for anything. you can find things to do ...or you can do nothing at all.
we were there the last week of february...the weather ranged from low 70s to high 80s...didn't get any rain....but some very windy days.
don't think you would be disappointed...it's not commercial and most definitely not crowded!