While not a large archeological site, Xunatunich is an interesting and pretty spot, and has the added bonus of easy access.
To get to Xunatunich from San Ignacio (Cayo), take one of the numerous west-bound buses. Tell the driver you would like to get out at the ferry to Xunatunich, which is about 7 miles west of town. The bus ride should cost US$0.75 per person. The ferry across the Mopan River is free. After crossing the ferry, there is a one-mile uphill walk to contend with. Bring drinking water!
You may hear from local people, especially taxi drivers, that it is unsafe to walk this stretch. They may try to scare you with stories of muggings or worse. My feeling on this is that it is a ploy to get taxi fare, but you need to decide for yourself. I did not find the walk to be threatening in any way, but I was with my husband. Single women should maybe hook up with other travelers to make the trip.
After paying the US$2.50 per person entrance fee, we stopped briefly at the unimpressive museum. Fortunately, the site itself did not disappoint. The site is a combination of excavated and unexcavated ruins, the largest restoration being a 130-foot stone pyramid called El Castillo. As Xunatunich is already set on the top of a large hill, the view from the top of El Castillo is long, and includes 360 degrees of seemingly endless jungle. Also from the top, the view of the rest of the archeological site, including the courtyards of green grass and families having picnics, is very tranquil. El Castillo has some ornate carvings on the east and west sides. While beautiful, it was interesting to find out that they are replicas, the originals being kept elsewhere for safekeeping.
While exploring the lower structures and hills (which contained unexcavated structures), we were able to find private quiet corners for relaxing, and we saw a lot of bird life. I lost count of how many parrots and toucans that we saw flying back and forth across the courtyards.
We spent ½ day traveling to and from, and exploring Xunatunich and felt that this was adequate time. I suggest visiting Cahel Pech in Cayo first to get the benefit of its superior museum before visiting Xunatunich. One could be visited in the morning, and the other in the afternoon, with lunch in Cayo in between.