Arya saves over a hundred Northern prisoners at Harrenhal with the help of Jaqen H'ghar in "A Clash of Kings." Arya uses the threat of his name to demand that he free all the northern prisoners at Harrenhal. H'ghar agrees after objecting at first, and they kill the guards in a funny chain of events involving a lot of hot soup from the kitchens. Her actions essentially take back the castle for Northerners.
Because she uses her name threat to save the Northerners in the books, and not to have Jaqen help her escape, as on the show, she has to escape all on her own. After she finds out Roose intends to leave her with the Brave Companions, led by Vargo Hoat (he's the guy who cuts off Jaime's hand but is named Locke in the show) at Harrenhal in "A Clash of Kings," Arya decides to escape with Hot Pie and Gendry. She steals a dagger and map from Bolton's chambers (the map part is really important), and then kills a gate guard, before the three of them ride out. Pretty clever for a girl who's supposed to be only 10.