Xlapak (8 a.m.–5 p.m., free admission) is the smallest and most skippable of the ruins on the Puuc Route [1]; indeed many tours don’t stop here.
That said, its single restored building has ornate facades on both sides, including some very well-preserved Chac masks.
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/the-state-yucatan/the-puuc-route