Las Grutas is in all guidebooks (you know, the ones fro travellers like Lonely Planet, Rough Guide, Footprint).
My secret: Prachuap Khiri Khan and Koh Kood, both in Thailand.
In Argentina:
America (called "Rivadavia" in some maps) in the BA province for that Pampas feel (the perfect little pampas town);
The road from Venado Tuerto to Junin through small towns for the "triangle of fertility" (the best land of the pampas).
Alta Gracia Golf (and "Che Guevara´s"!) neighborhood.
The road from Fiambala to Chile (Catamarca) and the nearby gigantic dunes of Medanitos (that one maybe only in one guidebook or not at all).
Many to choose from...