Cancún [1] has three notable archaeological sites, two in the Zona Hotelera (El Rey [2] and Yamil Lu'um [3]) and a third north of town (El Meco [4]), near the Isla Mujeres ferrfy. None compare in size or wow factor to the Yucatán Peninsula’s major sites, but they are still worth visiting, and the Zona Hotelera ones can be easily combined with a day at the beach.
Links:
[1] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun
[2] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun/sights/archaeological-zones/el-rey-archaeological-zon
[3] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun/sights/archaeological-zones/yamil-lu-um-archaeologica
[4] http://www.moon.com/destinations/cancun-the-yucatan/cancun-cozumel-and-the-riviera-maya/cancun/sights/archaeological-zones/el-meco-archaeological-zo