The psych department or person assigned to Natasha when she joins SHIELD complains a lot about stupid things in Natasha's opinion, like the fact that her room/s at SHIELD/the hellicarrier aren't 'personlised'. Clint tries to help her out by sticking a poster up, a movie poster or something, Natasha retaliates, and then The Great Wall Covering War is on. Their walls become covered with funny posters, rude ones, meaningful, postcards they bring back...then maybe ticket stubs, maps, scraps, memories...

Cover those walls!