No I don't think PdC is a shithole at all. Sorry if it came off that way. It just grew really fast and got busier, more trendy, etc. Didn't mean to make it sound 'bad'. Sorry! Seems like that eventually happens to all the good spots. It's just not our thing any more. AFAIK it's still nice, with lots of great options, but we don't know it well anymore, at least not beyond the very central area. I'm sure some of the other guys have more recent experience there.
Re: ruins, the Tulum ruins are great, esp if you can take some time at each structure to learn about it. The guided tours there are awesome. If you're looking for a day trip, Chichen Itza is amazing, too. Cenotes: There are so many, and new ones have opened every time we go back, so these are just a few of lots of options. We love to relax and jump off the cliffs (only 10-15' feet high) at Cenote Azul and Cenote Garden of Eden/Jardin del Eden. You'll see the signs for those off the main highway on the right as you go south toward Akumal.
Dos Ojos is a bit more involved. It's a cenote with one larger opening and a a smaller one 50-100 yards away. You can actually float or dive underground from the big one to the smaller one, thorugh the rock formations. People come from all over the world to dive it, but you can also get a guided floating/snorkeling tour. Everything is marked and heavily traveled. They know the way. Everyone has a light, life preservers, etc. You can also just get in the water at the big opening and just hang out there.
We just discovered Cenote Taak Bi Ha last time we were there. Amazing. One little tiny entry, then down some stairs, and you're in a completely underground, circular cenote. This is just a portion of it. Maybe 1/3 of it. It's very near Dos Ojos. IIRC it's on the same little road that you turn onto to get to Dos Ojos: