We went last year, thanks to the generosity of my inlaws. I've never been to an all-inclusive resort, let alone done any significant vacation travel (aside from car trips).
There are several tours. We really enjoyed Rio Secreto, which was a tour of a cenote and a long underground river. You wear a wet suit and life jacket and walk/swim/float all the way.
Another excursion I really enjoyed was called Mayan Adventure, or something like that. You travel to a small Mayan village that has partnered in the business venture with a tour company. You get to repel down into a huge cenote, hike to the other side and then zip line back. You also go to see the pyramids at Tulum, which are larger, I believe, than the ones at Chichen Itza.
As well, you go on a kayak trip and meet a Mayan shaman who purifies you before entering a rather special cenote and then kayak back and sample their local liquor. It is our tradition in Canada to bring a tobacco offering when meeting an elder, so I bought some really nice cigarillos and presented them to the shaman. He was surprised and seemed quite moved, and later presented me with the crystallized tree sap he uses for incense during the purification ritual. So, you might want to bring a gift out of respect, although no one else will, most likely.
When we were at the pyramids at Tulum, we were pressed for time and my back was quite sore, so we rented bicycles. You can also rent what they call Mayan limousines, which are a form of bicycle-powered rickshaw with a man up front pedaling and a bench seat at the back for the passengers.
Although I was in heaven with the food (as much meat, especially pork, as you want from the buffet, I was more enamoured with the Mayan people. Granted they now make their living off of tourism, I found them more decent than other societies heavily reliant on tourist dollars. I found them to be a humble, yet proud people and, once you get past the "me tourist, you want my money" relationship and actually spend some time with them, I hope you too feel the warmth I experienced.
If you've never traveled to Mexico, read up as much as you can. Although the black market still accepts foreign dollars (we were surprised to see they preferred Canadian dollars over American - apparently, Canadians make up the majority of tourists in that area), it is actually illegal for them to receive foreign dollars and can be difficult for them to trade them back to pesos, or so we were told.
Tipping in Mexico is customary. Learn the appropriate amount for specific circumstances (luggage handling, room maid service, information, etc.). We left good tips for housekeeping and seem to get a few extras and lots of genuine gratitude. I get the feeling some people who go to all-inclusive resort are poor tippers.
Haggling is common, but if one offers too low a price, one could easily seem greedy or insulting. Talk to hotel staff or tour guides to get an idea of when and where to haggle, how low to start, and when to pay full price.
I was surprised when we landed at the Cancun airport, to see all of the various agencies offering to take you to your hotel. They are all lined up in the airport near the exit. Don't go with them. Before getting to your hotel, they will take you to see some properties and try to sell you a time share. Walk past them and find your bus outside. Tip anyone who gets you to your bus, as they may just have saved you some aggravation.
If you're like me and never seen an iguana in the wild before, prepare to be amazed. They are everywhere. Some of them can be quite vicious with each other, so don't get between any that are heading toward the same piece of food. Also, monkeys come out as the sun goes down, so you might get to see a few in the coconut trees.
The resorts will also try to sell you a form of time share. These actually are pretty good deals if you travel much. The idea is you purchase a few years worth of vacation time that can be used at any of the destinations the company has around the world. Since we barely have enough money to pay our mortgage, we weren't interested, but we did do the math and were surprised to see how much one could save if they vacation each year anyway.
If you enjoy shopping or the night life, Playa del carmen is quite pretty (in a touristo kind of way). It's North of the resorts but South of Cancun. To save money, we took the colectivo (a van that runs up and down the major highways as part of a public transportation system) from the resort to Playa and it was quite affordable.
Plan your trip as much as possible before you leave. You will save quite a bit of money if you pre-purchase your excursions.
Finally, if you can afford it, buy decent flip flops and water shoes. The coral on some of the beaches can be quite sharp and, if you get the ones that cover the ankle, you are less likely to get sand in them. Well worth the investment, we found.
I really hope you have as wonderful a time as we did.
Gonzo
Edit: my partner just informed me, the pyramids we ventured ro were at Coba and not Tulum. My bad.