Hi there tsc2012,
Our agency has dealt with almost every company down there so I certainly have recommendations based on good and bad experiences!
I highly recommend using "Your Cancun DMC". They are punctual, experienced and have very knowledgeable guides. On top of that, their pricing is very competitive as well. Their website is http://www.yourcancundmc.com/
They will be able to help you with Chichen Itza, Coba, Playa Del Carmen, whatever. They also will assist in emergencies (one of my guests lost her Mexican Tourist Card once and they sorted everything). The best thing is their turnaround time. They actually respond!
I also highly recommend doing it before you get down there... Trying to arrange a group excursion when half your guests are at the pool bar, some are in their rooms, others in restaurants, honestly you'll waste half a day trying to sort everyone. Do it in advance or if you have a TA ask them to do it. Your Cancun DMC will pay your TA a commission so most TA's will be happy to work with them.
Hope that helps!