I *do not* recommend Cancun, but rather a nice area just south of Cancun, much more lowkey and no less lovely called the Riviera Maya. It began developing probably 10-12 years ago, maybe starting with Playa del Carmen and then it just grew from there. It's a great place now, with many lovely resorts and just a great place to relax, grab some beach time, have happy hour, well, whenever you're happy, and just enjoy your surroundings. You can also venture on up to see some Mayan ruins at Tulum, a very nice destination for some real Mexican history. You don't have to worry about it if you're not much for the water, just sitting by the beach is so "hypnotic" and "mesmerizing" that you can lie down, sit on chairs if you have a balcony room on the beach, or maybe just walk to waters edge and dip your toes in the ocean. It feels sooooo good!
Add on some great food, service and local culture and you'll be quite satisified with the Mayan Riviera. Cancun is WAY too crazy now......I'm about your age and I was in Cancun when it was still being built and was basically calm. If someone gave me a FREE vacation to Cancun, I'd not take it. It's way overdeveloped, full of themed restaurants and full of people who aren't too sure of their limits for alcohol intake. Unless you booked yourself into the Ritz Carlton in Cancun, I would not go there.
Here is a link to the official website of Riviera Maya so you can have a look around and see for yourself: http://www.rivieramaya.com/
And in advance if you come across a resort called Riviera Maya Akumal and wonder how it is, it's great. Many of our travelers on FVC.com and IT.com have visited this resort but as you'll see there are many. We just seem to get a lot of inquiries about that particular resort!!
Hope this helps get you started. See what you think and come on back with your questions or comments! We're here to help, as always