The Ninja House is a house located in Kobe which Chihiro and Oda visited during the school trip. 
Daiki tells Chihiro and Oda that they can leave since Mao seems interested about the things one store is selling, implying that Mao will take a long time. Chihiro invites Oda to go with her to find the "trickster" at the ninja house. To Chihiro's surprise, Oda accepts Chihiro's offer.
At the ninja house, (which seems to be a maze) Oda and Chihiro search for "contraption" hidden in the room in order to get the door open. Oda and Chihiro bonk heads in the process of finding the contraption, and they yell at each other in annoyance. They look at each other with an awkward silence. Chihiro mentions aloud that it has been awhile since they have talked like this, and she comments that she's glad that they are friends again, since she thought she messed things up. Oda tells her that she has been holding back, and says that he wouldn't want to lose Chihiro as a friend. Chihiro is surprised that Oda cares about her. Oda manages to open one of the hidden doors. Chihiro mentions to Oda that it would be fun if they could go to the fireworks festival again. Oda tells her that Chihiro is an oddball since she still talks with him even though he rejected her (unlike many of the girls who confesse to him), so Oda accepts Chihiro's offer.