We were in Cozumel in 2001 and The Mayan Riviera (near Tulum) this September.
If you can spring for it, I'd recommend flying to Chichen Itza (I have no idea why that wiki site has it as chitzen itza). We flew from Cozumel to Chichen which was awesome. It was a domestic airline - I don't happen to remember the name. The pilots did a fly by of the site so we had a great aerial view. Best part was we were there first thing so there was basically nobody there. We were finished by noon and got taken to a nearby hotel for lunch. As we were leaving the buses from Cancun were arriving with the hoards of tourists.
Besides that, there are a billion things to do down there, it just costs money. When you arrive in the airport these helpful guys in blue shirts will whisk you over to their counter and give you a map and circle a bunch of sites and then try and sell you on visiting the timeshare resort they are pushing. It's all pretty low key from our experience and if you were wanting to do the various tours they package together their prices are good. They basically take Tulum, Xcaret, and one other site and give you enough off the rates you get one of the tours free. If you're not into that just politely decline, and they won't pressure you. Note, the various tour companies are situated outside the terminal so you can bypass the helpful guys completely.
Whoever your charter is with (Nolitours, Air Canada, Sunquest, etc) will have an on site coordinator at the hotel and will have packages to all the major sites. The coordinators are typically Canadian or at least one of them will be. They do have a bit of a premium over what you could get going direct but it's not horrible in our experience, it might be $5-10 bucks more.
If any of you dive, use vip-diving.net. Canadian operators, really good. They take max 6 per boat, not 20+ like a lot of the resorts do.
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