We used Gray Line for our Chichen Itza tour out of Cancun--large tour bus group, but the guide was good.

Also look at ecab explorers based out of Puerto Morelos.

I've only used them for airport transfers but others have used Entertainment Plus with good results, been in business over 14 years.

Thanks for both your replies. I will look into both. I've also done more research and would consider renting a car and going without the tour...have read many comments on both sides of this. The expensive ($20)toll would be a non-issue if compared to the cost per person or using a tour operator...do the major hotels rent cars out of their properties or do I need to go into town to do this? I'm so NOT comfortable with doing this in a foreign country but it might be the option that makes the most sense financially and time-wise.

Renting a car is usually easy from a major hotel in the zone (or rental agencies will shuttle you), and the toll road is quite straightforward, a very easy drive. You could also include Ek Balaam if you left early (see chichen early, have lunch, then Ek Balaam, a wonderful site, easily "doable" in an hour or two: http://www.locogringo.com/yucatan/ekbalam.html; about the same entrance fee, give or take).
10 pesos to park at CI, ~100 (~$8) pesos entry fee pp. You don't need a tour guide for either site; I've been several times, and enjoy it much more without a guide.
I highly recommend having lunch at the Mayaland hotel, within the Chichen site (http://www.mayaland.com/hotelmayaland/restaurants.php#content)

Agree with the comment about car rental. If your hotel doesn't have an agency, they'll undoubtedly be able to recommend a way to get one easily. I've rented in Cancun 3 times, and I have found driving in mexico to be easy. The only caution is that slow scooters share the road so you have to watch out for them.

As of Feb. 2012, the entrance fee is 177 pesos. http://wikitravel.org/en/Chichen_Itza#Buy

The best part of renting a car to go to Chichen Itza is that you could rent the car late in the day on the day before you go to C.Itza, so you can leave early in the a.m. to get to Chichen Itza. this way you can get your touring done at Chichen Itza before the TOTAL ZOO of thousands of people get there when the tour buses roll in around 10:30 to 11 am. And before the hundreds of really obnoxious hawkers set up their wall to wall tables of trinkets on the paths that go from ruin to ruin (they sell these awful jaguar call devices and they all blow them nonstop from their table, it is awful).

I can not emphasize enough how important it is to get there early before the zoo atmosphere starts. I have done it both ways, getting to Chichen Itza early and being there almost alone, w/few people there from 8 a.m, til around 10:30. It was wonderful. But I have also gotten to Chichen Itza around 11 a.m. or noon and it is a totally different deal then. It was so crowded (the entire parking lot, which is huge, was full of tour buses) and such a zoo of noise w/the hawkers crammed in their tables which are lined up on every inch of available sace on every path through the ruins, that we literally went inside the gates, walked around 10 min. and turned around and left. It was AWFUL.

I have rented 3 times from Easyway, they will deliver to your hotel and their can give you a quote to include all insurance including zero deductible, reliable and nice folks.

EasyWay is great--very nice staff, reliable and economical cars, reasonable prices. The toll road from Cancun to Valladolid, and on to Chichen Itza, is very good, and we fouod very little traffic at all on it (a few weeks ago). The actual toll is a minor thing IMO. I understand that you are at an all-inclusive, but maybe consider one night in charming Colonial Valladolid to get an earlier arrival at nearby C.I., and also making Ek Balam an easier possibility. Ek Balam was (of C.I., Coba and Tulum also) our favorite and the pyramid can be climbed there if you have the stamina. The cenotes in the area are wonderful.

Hotels Arqueologico and Mayaland are also great choices, virtually on the site; you could visit in the evening (and early morning), and still do other stops on the way back.

Thanks everyone for great advice. I will definitely look into EasyWay, especially if they deliver directly to the hotel. I'm wondering if Xmas day will be less crowded there or just as bad as any other day since it's during the holiday vacation. About how long a drive would you say it is to Chichen Itza from the hotel strip in Cancun? If I can figure out my timing, I can calculate what other sights we can take in (while at the same time realizing my 16 and 18 year old would likely much rather be back at the hotel swimming or playing volleyball).