Stayed at the Coral Beach 15 years ago, and it's beautiful. You can't replicate Mexican luxury like that in the US. Was there this past summer, two nights at the Omni, 5 nights in Playa del Carmen. Top rec in Cancun: Lima 1205. Make sure you get the Ronda Fria.
Other than that a trip to Isla Mujeres to walk around and get away, and Puerto Morelos for snorkeling. Puerto Morelos was incredible, most of the reef was only 6-10 ft deep, many parts you're literally skimming above the reef, very unique,and cheap. We got lucky and had a solo tour for $25 pp. Just walk up and negotiate there at the dock. I'd also recommend bringing your own snorkeling gear. It's pretty gross to use the house stuff. Xcaret was great 15 yrs ago, and Xplor gets great reviews, but both are a bit too expensive and Disneylandy for my taste.
Obviously the ruins are a must do, but you already know that. Wish I had woken up earlier to do Coba. I'd recommend renting a car for some freedom. We found a ridiculous $3 a day rental, which came out to $15/day after loading up on insurance. Really worth it to explore Valladolid and smaller towns to and fro Chichen Itza.
If you're sick of the tourist food, Vips is across the street for breakfast. It's a very good Mexican chain and where you'll only find Mexicans eating.