ADO makes a stop in Coba, but I don't know if you can get there directly from Cancun or whether you need to go through Valldolid or Tulum to get there. Here's their site: http://www.ado.com.mx/ado/Destinos.jsp There are tour companies that take groups there. One of the good ones is Alltournative: http://www.alltournative.com/tours-products/maya-encounter
If you are really into the "ruins" Valladolid is about 20 minutes from Ek Balam, 30 from Chichen Itza and 45 from Coba. There is public transportation (bus) from downtown Cancun to Valladolid. The bus station is 3 blocks from the town square. I think the best hotel is Meson del Marques. Then you can also get public transportation to all 3 of tghe nearby "ruins" but they are all in different directions. I am pretty sure that there is only one bus back to Valladolid per day and it leaves Coba around 12:30 or 1:00. So if you do this gig, go early as you have to leave early or check for taxi fares. My guess would be $20-$25 US maximum to get back to Valladolid.
