I would echo the recomendations for Dennis Weeks at Diablo (and now also Aquanuats). Dennis is originally from Alberta, Canada and is excellent both as a Divemaster and an instructor if you want to obtain additional certifications. Depending on where you are staying, he can pick up and drop off at your hotel. His dives will maximize your bottom times and, if you don't have your own gear, he has reasonably priced rental gear. I dove with him last fall and will again this November. One of his specialties is Cenote diving and, if you have never done this, it would be worth your while to spend one day doing a couple of these fresh water wonders. I do not recommend diving with the operators who work out of the major resorts as they maximize their revenue by doing quick turn-arounds. Dennis will be more economical and provide a better experience for the two of you. As for ruins Tulum is worthwhile but, now that it has made the list of The New Seven Wonders of the World, Chichen Itza would be the place to go. If you had the time Cozumel would also be worth a day trip and Dennis is working with a couple of Coz shops (Living Underwater is one and Jeremy is great) and can arrange pick up and drop off from the Ferry dock in Cozumel. Have fun.
