Uxmal is one of the major sites in the Yucatan and was my favorite. It's worth getting there early before it's hot and crowded, but I doubt it's ever as crowded as the Cancun-accessible sites.
I loved everything about the sites we visited in this area, but Uxmal combined all of the good parts into one perfect site. You can still do plenty of climbing around, the decoration is wonderful, and there's a great pyramid. I know Chichen Itza is the site to visit in the Yucatan, but this is the one I want to go back to.
I want you to know that I am showing great restraint here in the number of photos I'm posting.
Also, there are little paths leading off to other ruins. Be sure to poke around.