Uxmal is a major Mayan site. It's the ruins of one of the key Mayan cities of the Yucatan. Uxmal is the best-restored site along the Puuc Route, the string of Mayan cities in the hills south of Merida. I won't try to confuse things by making too many mistaken claims about the history here, but Uxmal peaked a little before the better-known site of Chichen Itza.
The first part of the site you see after you come in the gate is the very impressive Pyramid of the Sorcerer. But there are several other equally stunning temples and pyramids on the site. Unfortunately, tourists are no longer (at least in 2003) allowed to climb the Pyramid of the Sorcerer, but that was the only structure on the site that was restricted (this isn't unusual - two of the major structures at Chichen Itza were closed off, either permanently or for restorations).
Don't miss the nightly sound and light show, viewed from the wall of the "Nunnery Quadrangle" at Uxmal. It's amazing to be there after dark, and the show is entertaining. If you have very sensitive kids, they might be a bit scared by some of the chanting on the audio portion of the show, but it isn't really that bad. The show here is more theatrical and less factual than the one at Chichen Itza.
We didn't hire a guide here, and didn't miss it too much. We had hired one at Chichen Itza and felt totally ripped off. The stated rate is a little lower here than at Chichen Itza, but we still didn't think it was worth it and just read descriptions from our written guide materials.
I wandered off (without the kids, who were getting hot and tired) to some of the more off the main path structions, such as the platform of the stellae. They weren't easy to find and weren't really very memorable.
In a lot of ways, we liked this site more than Chichen Itza. There are just as many major structures, and a lot fewer people.
Other recommendations:
Wear a hat! Bring water bottles! Buy an ice cream bar back at the restaurant and cool off.
Shopping
The museum shops at Uxmal and the T-shirt vendors outside are about all you'll see for souvenirs across all the Puuc sites. There were a few postcards at Labna and Kabah, and a small cafe at Labah, but that's about all - nothing at all at Mayapan. It's the opposite of the "try to sell you something through the window at the speed bump" environment of Chichen Itza.