Everybody loves a "free gift", and there are lots of them in Cancun. Just remember, "there ain't no such thing as a free lunch".
You might be offered free lunches or free breakfasts in exchange for listening to a sales presentation. The consequences are explained in the section on time-sharing companies. If you go to a reputable presentation (that is, affiliated with a major hotel), and you know that you can resist the sales pitch, you actually can get some free or discounted stuff. I found the discounts on the tours to be the best part of the deal, saving about $50-60 US over 3 tours.
You will be given various coupons for free admission to clubs and discos, or "2 for 1 cover" coupons. However, if you check the signs along the upper hotel zone each afternoon, you'll find that most nights are "no cover" nights at most places, or "no cover between the hours of...". You might even see "no cover" coupons for places that never have cover charges!
Since cover charges can run to $10 US, it is a good idea to save on them but the coupons don't help a whole lot.
These coupons, quoting "all you can eat for $X", usually state the same price that everyone gets. Places that use these coupons are not likely to be the cheapest either, so consider these a signal that there is a better price elsewhere.
Aside from the silly coupon business, the all-you-can-eat meals are a good deal and there is usually good variety and fresh food.
If you like XYZ, go ahead and take advantage of these.
Each time you turn money into pesos, you are losing something on the transaction. Because the US dollar has the best exchange rate, you can save money by paying most items with US dollars -- even if your money was in some other currency to begin with. For an explanation, see the section on exchange rates.
Many restaurants give a 10% discount coupon with your bill to get you to return. See the section on meals for the restaurants I found that did this.
As you walk down many of the side streets, greeters for the restaurants will offer you free drinks to have a meal there. If you happen to be hungry and the place looks clean, why not? Food is cheap everywhere and you will tend to get very thirsty anyway.
Not being a woman, I can't comment on how valuable these are. A lot of clubs designate some nights as "girls drink free". Other nights feature bikini contests with cash prizes. Considering the low women:men ratio of the places where I went, I feel like encouraging more women to take advantage of these offers. :-)
I didn't find the coupons given out by the bus drivers or in the "money saver" books available on the bus to be of any value.
The discount coupons given by restaurants with the bill have no catch -- if you liked the food, it will be 10% cheaper next time.
The best places I found for coupons that you can use were the same places that tried to get me to go to time-sharing pitches: information booths and tour booths in malls. Mixed in with the bogus coupons were some that I could use.