These are known as sanitarios or baños. They are damas(women) and hombres or caballeros(men). Public rest rooms are hard to come by. You will almost never find a public rest room in a city park or downtown area. There are usually rest rooms at most national park entrances and much less frequently inside the national park(there are usually plenty of trees and bushes). In the towns and cities it gets much trickier. One must count on the generosity of some hotel or restaurant. Same goes for most beaches. However, most restaurants and to a lesser degree hotels, will let you use their facilities, especially if you buy a soft drink or something. Bus and gas stations often have rest rooms, but many of these are pretty grim.