Use the morning for more Buenos Aires time, with your leader available for suggestions if you want a neighborhood, cafe or market idea. Later, cross the Rio de la Plata by ferry and arrive in Colonia del Sacramento, one of Uruguay's oldest cities. The World Heritage-listed old quarter is the obvious first stop, with quiet lanes, Plaza Mayor, small museums and river views. If you want a better look over town, climb the still-working 19th-century lighthouse.