Of course, with your having spent the millenium in Belize I would rule out the Yucatán (meets all your criteria but would be too similar to the Belize trip).

So, I would suggest the Pacific coast. One option would be Oaxaca. You could spend 3 or 4 days in Oaxaca City seeing the ruins of Monte Alban and Mitla, visiting the various Zapotec and Mixtec towns within the valley, shopping for the wonderful handicrafts and folk art, and absorbing the Spanish colonial history of the city. Then for the next 5 or 6 days you could stay on the coast at Huatulco, Puerto Angel, or Puerto Escondido. There all just a short 45 minute flight away.

Further north, you could do a similar trip visiting the Bajío cities and towns of Guanajuato and San Miguel de Allende. After a few days you could take a deluxe bus out to the coast and either stay at the more developed bays of Manzanillo or go a little further north to the quieter lovely bays and beaches of Barra de Navidad, Tenacatita, or the Costa de Careyes.

A third possibility would be a train trip through Copper Canyon and then a charter boat cruising the Sea of Cortez from La Paz north along southern Baja's east coast.

All of these (and more) close by in that nation directly south of us. John Rule San Diego, CA