The third archaeological site I went to during my visit to Playa del Carmen in March 2012 is Cobá, a largely undiscovered site which is about two hours drive away from both Cancun and Playa del Carmen.
While Cobá is not as popular as Chichén Itzá, it makes for a great day trip. Main differences I immediately noticed between the two archaeological sites are: 1) the structures at Cobá have not been restored as thoroughly, which somehow makes it feel a little more undiscovered; 2) only a small portion of the structures at Cobá have been unearthed, while many are still waiting their turn, hidden under the overgrowth of the jungle; and 3) The main pyramid at Cobá (Temple of Nohoch Mul) can still be climbed… bonus!
The land area of Cobá is huge, so the best way to get from temple to temple within the site, is by bicycle; you can rent a bicycle at the entrance to the site for next to nothing; biking through the jungle makes the experience even more entertaining!
All in all, I really enjoyed Cobá, and while the site is not as famous as either Chichén Itzá or the coastal Tulum, I highly recommend a visit to this archaeological site!