Were in Mexico would make a great travelin for backpackers?

I have been to Cancun in an all-inclusive and never learned a thing about Mexico except that the food was great at our hotel... This time I would like to get out there! Do you know a good spot? But I would have to be close to the beach and a few restaurant and bars won"t hurt either...! My friend told me about Playa del Carmen and Cozumel; aren"t these tourist meccas ...?