Thanks for the suggestions so far! In answer to your question Frank - No, we've never been to any major ruins yet. We'll be diving/snorkeling in Grand Cayman and doing the same in Cozumel. That's why we're considering Lamanai but would really like a chance to experience Ambergris also - so little time and so much to do! But alas, we can't do both.
It's a fairly long trip to Lamanai(about 45 min. on bus and another 1 1/2 hours by boat) to reach the ruins and then back again. If the trip there and back is interesting, and the ruins worth the visit then it might be a good choice. Since we've never done this type of thing before wasn't quite sure what to expect and if it was worth the investment of all that travel time.
Has anyone done both and is there a clear preference or is it like Sandy said - if we feel like having a beach and fun in the sun experience do one thing - if it's history and fauna do the other?
Let me know - Thanks much!