Now, for some of you, the constant barrage of sales pitches--"Hello, Amigo, come to my store!", "Where you from?", etc., is agressive, but take it in stride; they are looking to make a buck, sell you a tour, sell you souvenirs, etc.
I have said this before: be prepared to haggle. Now, it will be tempting to buy souvenirs and gifts from those boutique shops in Cancun, but it will cost you, and you can can find the same stuff cheaper. They have some really decent open markets that sell all kinds of stuff, from T-shirts to pottery, chess sets, coasters, etc.
We shop downtown; you just have to ask for "Mercado 28" or "Kiwi Market". They're similar, but I found the Market 28 more affordable and with some decent seafood restaurants nearby. Also, remember that if you need anything else, there is a huge Wal-Mart and Chadrawi supermarket for groceries, bottled water, disposable cameras, etc. These places are all in the "centro area" downtown and not hard to find--the good thing about Cancun is that bus transport is easy and only about 6 pesos. In general, everything is cheaper in downtown.