You are in an excellent spot to go to the Mayan Ruins. Belize has 3 different ones to choose from that you can easily do and be back to your ship in plenty of time.
Altun Ha is the easiest to get to, and it is very pretty. You can hike up two of the ruins for an amazing view.
Lamania is the one that impressed me the most. You will go on a river boat tour up to the ruins. Trust me on this one, you will be impressed!
Book online though, skip booking with your cruise ship. The cruise ships only moved into Belize a few years ago, and they have been trying to push the prices of tours up. The local companies are trying very hard to keep belize affordable. But, definitely book before you go as when you get off the ship, you will have all sorts of little guys offering you tours in their brothers taxi. You don't want to go without a set plan. Book ahead and skip the craziness.
I have used these guys before, great prices and a great tour.
Belize Lamanai Temples