Rocky Point is one of the few places where land touches the Belize Barrier Reef. It can be hiked to from the Tranquility Bay Resort. Unfortunately, we discovered the trek to be longer than it seemed. Not that we didn't try--I even destroyed a pair of flip-flops on the coral-clogged beach. If you want to make the hike, wear Tevas or other water-friendly hiking shoes.
You can also take a boat out to Rocky Point instead for some fishing or snorkeling; or even better, continue the voyage through the ancient Mayan canal that splits Mexico from Belize. On the northwestern side of Ambergris Caye, you can visit an unexcavated Mayan ruin, Chac Balam.
Ambergris Caye has Mayan ruins? A fair amount, in fact. Just take a look at this map of archaeological sites on Ambergris Caye.