I love Mexico, but Cancun is probably one of the last places I'd choose to stay. As others said, it is very commercialized and is not at all a good representative of the stunning Mexican culture. (Think spring break from hell).
So in your case I'd choose Hawaii - there is so much to do! Last time I was there (Maui) I scuba dived, biked down a volcano, took a surfing lesson, snorkeled, went whale watching and actually spent very little time laying around on the beach. (My then fiance also couldn't STAND just laying around!)
The drawback to Hawaii is that it is extremely expensive.
Congrats on your pending marriage!