After you leave the Burrow and have finished collecting Ginny's lost supplies, go around casting Perfect Flipendo's on all of the objects you can find. Collect the Sickles, if they appear. Once you have enough Sickles, enter Gambol and Japes. Purchase a 10-pack of Stink Pellets. These can be useful when facing enemies, or if you are caught by a Prefect after hours.