What sort of thing are you looking for?
Of the "resort" type places the Carribbean side (Cancun, Playa del Carmen, Tulum etc have the best beaches and crystal clear warm sea but it's not really an authentic representation of Mexico (although Tulum is pretty decent with the ruins etc)
Pacific side (Puerto Vallarta and surrounds) feels more authentic, but beaches and sea not quite as nice.
Holbox and Bacalar were recently the "in" places amongst my wife's well to do Mexican friends, but don't know much about them personally.
Other well known beach places are Los Cabos which is a place Americans go to die, and the surfing places in Oaxaca like Puerto Escondido which are geared more towards travellers. Huatulco is really nice and popular with Mexicans but unbearably hot.
Imo it'd be a shame to go to Mexico and just spend time in a resort, there is so much to see and do in the country.