Have a look at belizecruiseexcursions.com as they offer a great trip
Sun Kissed Snorkeling & Beach Stop at Goff's Caye
We did this one because we have a 6 year old and it was one of the few excursions we found that was great for kids. We really enjoyed it, Elvis's Reef is really nice and Goff's Caye is a dream. Small island with lots of palm tree and not much else except for beautiful sand and crystal clear water. There is a reef right there too just 3 steps off the beach. We only paid $40 for us and $30 for our daughter. It was 4 hrs which was perfect for us and dd.
We booked a tour with Reggies Tours for cave tubing and had the most wonderful time. This was the highlight of our cruise. Reggie and Timmy were wonderful hosts and guides. This is a must if you are going to Belize. Just email Reggie at regtours.com and he is very accommodating. Highly recommend Reggie's.