Rosario is located in the province of Santa Fe.
The Monumento a la Bandera is a fantastic place to start--enjoy a walk around the city center and explore the culture and the various shops.
The Parque Independencia and La Florida Beach are definitely worth the visit and in both these wonderful places you can really get into the great outdoors--there is hiking, horse riding, canoeing, and even bike-riding to offer. If a slower pace is what you're looking for, take a picnic--you may find a beautiful spot to spend an afternoon.