The plan is achievable but it reminds me of the movie, "If its Tuesday, this must be Belgium".
First I would be sure to have the mapchick's maps of the area.
Second assume your drive times are close to optimal times and figure you will spend a lot more time getting your car than you are used to in the states.
Third, the free road from Tulum to Valladolid is a Federal Highway. It is also the width of most county roads in the states. Don't get in a hurry, the topes will end your excursion if you don't respect them.
Fourth, renting a car, doing the drive from PDC to Ek Balam, Valladolid, cenote and getting to your hotel is a bit optimistic. I would stay in Valladolid and make a morning dive to CI.
Fifth, you will have the time to tour Tulum on your return, but Coba will be cutting it pretty fine. You really want to be off 307 and back in PDC by dusk.
Sixth, don't even consider driving unless you are well acquainted with the local "rules of the road".
Seventh, all archeological sites have free admission on Sunday (used to be the rule and not sure if CI is an exception).
I applaud your idea of stopping and getting plenty of water, you will need it. CI, Tulum and Coba can be brutal, At Ek Balam, they left a lot more trees for shade so not quite as bad. Pick up sandwich fixings as well since you will be eating lunches on the run.
Last edited by jeffandbecky; 03-17-2011 at 12:38 AM..