We did the dolphin swim at Blue Lagoon. It was about 2 years ago and my kids are still talking about it and watching the video. It's expensive but I can honestly say it is the best excursion I've ever done.
Another great thing to see is Atlantis. They have an aquarium there and the Dig. You can check it out on their website.
We did some shopping and went to the straw market, went over to Paradise Island and walked through Atlantis and then spent the rest of the day at Blue Lagoon.
Celebrity Century 1/09
Carnival Destiny 10/07
Carnival Destiny 9/06
Monarch of the Seas 3/06
Carnival Inspiration 8/05
Carnival Miracle 4/05
Carnival Inspiration 4/04
Sovereign of the Seas 4/03
I agree with Darcy on the places to see. The only place I would be careful is in the straw market. Mostly for the younger kids. It gets very crowded. Also watch for some locals who hand you handmade bead bracelets then get pushy about receiving money for them. Otherwise, its a great place to experieince.
i also totally agree with Darcy. The Aquarium/Dig at Atlantis is amazing.
If you want to spend some beach time you can get a day room at the Comfort Inn next door to Atlantis which allows you access to the beach at Atlantis, not the waterslides, just the beach.