Chichen-Itza can no longer be climbed, the pyramid is completely roped off since like 2 years. There were structural problems to have this ancient site cope with thousands of tourists daily and these were directly related to safety issues, which also touch the climbing, since there also a person died from falling the stairs...Having climbed the pyramid before, to me it felt magic to be up there and you are higher than all treetops and see to no end at 360...The light show was ok, however I do not think that this is available daily, you may check before planning so, the night show I definitely remember was at Xcaret, 1h from Cancun and 10 min. from Playa del carmen..great stuff.
Coba is a large area and still offers some structures to be climbed, a good idea is to take on of the bicicles to see all the site, walking can be hot, time consuming..a great Mayan Site!
Other Mayan Ruins from reachable from Cancun, depending on how much time you will be there, you got famous Tulum (1h 45min from Cancun), Muyil (2h from Cancun), even San Gervacio in Cozumel (minor site, 1.5-2 hours from Cancun). Depending on the time you also have the beautiful Uxmal and Izamal, Dzibichaltun in the Yucatans to Kohunlich and Chacchoben in the Costa Maya Area (4h from Cancun)...good luck!