I went to Cancun last year with my in-laws. The trip was sort of a gift from them, so we kind of deferred to their wishes for the most part - they go at least twice a year, so have "regular" places they have to go and they don't like to leave the hotel district either. That's just a disclaimer that I didn't get to experience too much, but I do have a couple suggestions:
Kukulcan Plaza - typical Mexican version of an American shopping mall, not too exciting, but there is a Mango and a couple little boutique-y shops that are alright, there's also a Taxco (silver jewelry) store, and a large quasi-department store called Luxury Avenue, which just has LV, Burberry, Cartier, etc. The prices didn't seem any better than in the U.S.
La Isla Shopping Center - I made a special trip here to go to Zara, the rest of it is not too exciting. It's all outdoors though, which is kind of nice. I'm not really into nightlife (I think I overdid it in college and thereafter), but I think there are a few bars and clubs here.
My restaurant tastes are quite different from my in-laws, but one night DH and I convinced them to go Aioli in the Le Meridien hotel and it was lovely.
Oh, there's also an Italian restaurant at the JW Marriott that was nice.