Congratulations on you anniversary!!! We just returned from Voyager for our first cruise and we loved everything about the ship.
We did the half day excursion with Captain Marvins in Grand Cayman and it was wonderful I would highly recommend it. It was also less expensive than excursions purchased through the ship. The web site is www.captainmarvins.com
Also did Tulum/Xel-Ha through the ship and really enjoyed that as well, I wouldn't recommend the combo though because you really don't have time to enjoy both. I would have rather done just Tulum or the Tulum/Beach Combo.
The only other thing I would recommend is Portifino's. We did this for our anniversary and it was well worth the $20pp. The only extra charge was for our bottle of wine ($23 for Beringers White Zinfandal) Make sure you save room and order the Tiramisu (sp?) best dessert the whole cruise!