This is where the 'or' option comes in. You can skip leaving the airport and take a jumper plane to San Pedro or grab a taxi then a ferry to the island. This would give you more time to live the beach dream in Belize and check out so many fun activities in the area before heading inland. By doing this you are opting out of visiting the Altun Ha Mayan Ruins, but you can always catch them on the way back from the Southern Coast!
There is another opportunity to visit ruins on this trip as well so don't feel like if you miss Altun Ha that you won't see any Mayan ruins while visiting Central America.
Once on San Pedro you can grab a taxi to take you to the Caye Casa or you can rent a golf cart for you stay to explore more of the town.