If you're spending more than 4 or 5 days in Buenos Aires, you might want to consider a side trip -- especially if you're visiting in summer, when many Porteños hit the beach resorts. Mar del Plata is the country's most popular stretch of shore, and some of the country's most famous musicians and other entertainers follow the beachgoers there. To describe Mar del Plata as crowded in summer is an understatement, as more than eight million people visit in December, January, and February alone. Nearby, the town of Miramar offers a quieter beach vacation at a slower pace.
La Plata is the capital of Buenos Aires Province, planned in 1880 along neoclassical lines. Full of diagonals and parks, it is an interesting and open city with various museums and other sites of historical interest, which you can visit year-round.
Just outside Buenos Aires's suburbs is the Tigre Delta, a beautiful complex of islands and marshland full of small bed-and-breakfasts, resorts, and adventure trails. You can make a day trip here on mass transit from Buenos Aires, or you can choose to stay overnight. It is busiest in the summer season, but most sites and hotels are open year-round.
The Pampas surround Buenos Aires, and here you'll find gauchos and the stuff of Argentine cowboy lore. The region's main town is San Antonio de Areco, about 90 minutes north of the capital. Few people stay in town, preferring to lodge at the surrounding estancias (19th-c. ranch farms), several of which are detailed here.