San Bernardino — "San Ber" to everyone who goes there — is the closest thing Paraguay has to a beach resort, on the east shore of Lake Ypacaraí 50 km from Asunción. It was founded in 1881 by German settlers, and enough of them built enough houses that the older streets still have a slightly Bavarian look under the mango trees.
The season is short and absolute: from December to mid-February the population multiplies, the lakefront clubs open, and half of young Asunción is here at weekends. From March the place empties and becomes a pleasant, faintly melancholy small town with a good view. Which of those two towns you want decides when you come.
The lake is the complication. Ypacaraí is shallow, about 60 km², and has had serious cyanobacteria blooms in several recent summers that closed the water to swimming. It has also had clean seasons. Ask locally before you get in — the kiosks and the Club Náutico will tell you straight, because their business depends on it.
Beyond the water there is not a great deal to do, and that is fine. Walk the Costanera in the late afternoon, eat at one of the lakefront parrillas, look at the old German houses on the hill. San Bernardino pairs naturally with Areguá on the opposite shore and with Caacupé 20 km inland, which is how most people fit it into a Circuito de Oro day.
Buses from Asunción's terminal run through the day and take about an hour and a half.