I don’t think Playa del Carmen is at all nice, too built up, too noisy, too…manic… but if you Google it you can find cool small hotel run by Italian people, can’t remember the name, give it a try. Caribbean is so beauutiful; but try further down the coast for more purity.
If interested in a city, try beautiful amazingly lively Queretaro, not especially touristy, easy bus ride from Mexico DF, safe, clean air, stay at Quinta Zoe, eat well, drink wonderful coffee, be sure to get your shoes shined in one of the many leafy plazas, they do a beautiful job, your shoes will make you very happy. And you can walk walk walk walk walk and people will talk with you and help you with your espanol.