I would avoid Cancun like the plague. It's like Miami south. Below Isle Mujeres is Playa del Carmen which I used to recommended whole-heartedly but after my last two visits there it's turning into Cancun Jr. I think there is still some sanctuary south of there near Tulum: lots of eco-tourism and Mayan remains. Although not on the coast Merida is a great city unspoiled by tourism. (if Hurricane Emily doesn't destroy it tomorrow). It's the capital of the Yucatan province and a four hour bus ride from CUN. Buses in Mexico are nicer than most flights in coach in the U.S. Merida is a great old colonial city boasting the oldest catherdral in North America and had the most millionaires per capita in 1900. Lodging is very affordable with 4* accomodations running around $50US. I recommend the Hotel Colon. A good breakfast buffet is around $3US. I have had great experience in Merida with Yucatan Trails travel agency. Ask for Dennis. Tell him The Juiceman sent you. (no kidding). He can arrange all your lodging and tours in the Yucatan including the Mayan Riveria (as the coast from Cancun to the Belize border is referred to).
Have fun!