From San Jose Succotz you can take the ferry across the Mopan River (tipping the ferryman is a good idea) to the Xunantunich site. You'll know you're there when you see a group of stands with locals selling everything from jewelry and bags to stone calendars and figurines. On the other side of the river there's about a mile walk uphill to get to the entrance. There are also several "gift shops" just near the entrance. The cost to get into the site is about US$5 per person.
Recently a small museum was added to the site that displays pictures from archaeological digs as well as a diorama of the site and quite a bit of information on the history of the area. It's right at the entrance, so stop in before going on. It only takes 10 to 20 minutes to walk through.
Once you leave the museum, you'll see the main area of the site. On one end there's a large stepped pyramid with beautiful carvings on it. On the other side is a building that served as the housing and kitchen area. In the middle there are several mounds and a ball court that looks incredible when the sunset hits it. The pyramid is the most spectacular part of the site. It's a long climb up, but the view is absolutely incredible. I sat on top of it for a good half-hour just looking out over the rain-forest canopy. You can even see the Guatemalan border from there.
I should mention that you shouldn't be startled by the guards there. They carry rather large guns and wander the site, keeping an eye on things. I wasn't expecting it and was quite surprised to see one when I got to the top of the pyramid.
This site was my favorite thing about Belize. I spent most of my time in the rain forest and being uncomfortable, so this was a welcome change. I've been told that it's not as spectacular as the nearby Tikal or Caracol (neither of which I've been to yet), but it's definitely less crowded. I think there were only 20 to 30 people there when I went. And my group accounted for almost half of that.