Kids will want to climb up El Castillo, a pyramid over 130 ft high (and the second tallest in Belize). On the east side of El Castillo is a frieze a (replica of the original stucco frieze) with the Maya sun god in the center. From the top of El Castillo are stellar views of the surrounding countryside, you can see all the way to Guatemala.
In the trees next to El Castillo, look for black howler monkeys
Check out the small museum, which has three crumbly stelae, incised with images of Maya rulers, decked out in ceremonial clothing and adorned with elaborate headdresses.
Tip: Part of the fun getting to Xunantunich is riding the ferry across the Mopan River. At certain times, when rains cause the river to rise, the ferry doesn't run.