You should not miss the downtown open air markets. They are basically flea markets selling every type of souvenir from Mexico that you could want. They have some quality goods, jewelry and marble items, clothing, typical touristy souvenir items, housewares, etc.
I wouldn't recommend the shopping malls, as they are not anywhere near what we have in the States. They are set up the same, indoors with many stores, but many of the shops are Cancun t-shirts, beach towels, and things along those lines. I tried to find a new outfit for dinner and found nothing suitable.