I second Gaz's recommendation of a night dive at Hol Chan. My wife and I were married on AC in November 2002, and we did a lot of diving while we were there, including a couple at both Hol Chan and Cyprus Canyons.
The night dive at Hol Chan was amazing. We got to see parrot fish lying on their sides fast asleep, an octopus about the size of a basketball, stingrays buried in the sand, sea cucumbers "in action"
, and the highlight: a moray eel on the hunt, and acutally nabbing a fish -- very cool!
I also highly recommend Shark Ray Alley. It's a snorkelling trip in about eight feet of water. You can expect 5-10 large stingrays and nurse sharks around your boat at any given time, and there are always several boats in the area. Also, lots of fish -- big jacks especially.
One note about SRA -- be sure to remember what the guide tells you about making your hands into fists when the sharks swim up to you -- I've got a great set of scars on my thumb from an "incident"....but my wife thinks they make me look more manly, so you might want to get a set of your own...
Matt
PS: And Capricorn is truly amazing. Be sure to take the water taxi over when the full moon is out. You won't regret it!