The absolute best excursion in Grand Cayman is Captain Bryans Stingray City tour!! Its on an amazing catamaran so you get the stingrays and the snorkeling in addition to sailing one of the most serene areas of the Caribbean. You can't miss with this one.
You can book them on http://grandcaymancruiseexcursions.com
Also, you have to see the beach, seven mile beach it extraodinary, its wonderful