Look up Lomas online
Transportation, Tours & Hotels in Cancun Mexico: Online Reservations They book the trips through most of the hotels on Coz, or used to.
It's been several years since we have taken an organized tour, (I go on my own nowadays), but we booked both our Chichen Itza tour and our Tulum/Xel Ha tours through them at the activities desk at La Ceiba when we stayed there.
the Chichen Itza tour we did was the flight tour where your fly from Coz to Chichen Itza "airport", which is more of an "air strip", then you get in taxis to the archeological site, about 5 min away, and a guide gives you a tour of the site. You get much more time on site then you do on the bus tours.
the tour to Tulum we did was a bus tour. First we went to from Playa del Carmen to Tulum for a guided tour, the bus stopped once on the way down at a road side souvenier shop for "pottty breaks" and such. Then they took us to a resort along the coast for lunch and then up to Xel Ha for some snorkeling. Xel Ha was VERY dissapointing, the vis was only about 7' and not much to see. We would have much rather spent more time at the archeological site.
May I make a suggestion, for about the same price as a tour you could get an early start, (you will for the tours anyway), take the ferry over to Playa del Carmen and rent a car, then drive yourself to Coba first.
Hire your own personal guide for about $20-25 to take you through the site. You should still have time, if you are up to it to make Tulum and do the same thing. Sometimes you can sort of "blend in" with an organized tour group and get some of thier tour guide's info, but Tulum's buildings are very well marked and a guide is not necessary there.
On the road between Tulum and Coba there are 3 small Mayan villages, in the second one, I belive it is, you'll find a Mayan style hut along the road, (on the right if you are headed to Coba), with Mayan heiroglyph carvings out front. Skip the "tourist shops" and stop in there for some very unique souveniers to take home.
The artist that owns the shop takes real Mayan heiroglyphs out of archeological site books and carves them into limestone slabs, as well as making small replica ceremonial statues and other replica mayan artifacts. I have stopped there several times and own some very nice pieces he has created.