Talk to a Hufflepuff in the corner of the Entrance Hall and he will tell you he's looking for Five Wizard Cards. Here are the locations.

1. Top of long flight of steps leading to the boathouse.
2. On Grand Staircase near the bottom. From the Entrance Hall, go up the short flight of stairs to the Grand Staircase. Go down, left, right and right and the boy is there.
3. On fourth floor near where you found Nearly Headless Nick.
4. On third floor go to bottom of stairs and there is a Gryffindor in the corner.
5. between Entrance Courtyard and Viaduct.