Do you speak Spanish? The on-site guides at both Chichén Itzá and Uxmal were great when I went a little over a year ago. Chichén is huge, so I'd recommend budgeting at least 4 hours if you want to see the whole site and explore it in depth - get there as early as possible in the day to avoid the tourist hordes and the schlock tourist merchants that set up shop everywhere and really detract from the experience. Uxmal is much less crowded and can probably be done in just a couple of hours.
Another site I really enjoyed when I went was Dzibilchaltún, located north of Mérida. It was virtually empty, I negotiated a good price with a guide for a private tour, and I really wished I'd have brought my bathing suit to hang out in the cenote for a bit.