Hidden accessories
When you go to Pal park and Migrate pokemon from ruby, sapphire, leafgreen, or firered, after you are done catching all the pokemon, when you are inside the HQ of palpark go to the 2nd floor and talk to the lady in the top-right corner and she'll give you 2 new backdrops(ruby and sapphire only) or 2 new accessories(leafgreen and firered only).