I figured I'd beat him to it. My wife and I didn't have a traditional honeymoon. We basically lived it the past ~30 years together.
It depends on what you like. Downtown Cancun is very loud and touristy, although we stayed at this Hiton resort last year and it was great. Playa del Carmen is nice, but actually further south is Tulum and that is even better. Less tourists, more affordable. Great ruinas. Also, across the water is Cozumel and if you like to dive or snorkel, it is a must. Also, check out the cenotes down there. These are underground lakes the Mayans used. Fresh water, amazing. Here is a picture