The Gold & Silver versions have a new feature called Mystery Gift that you may use to get various items if you have a Gameboy Color. When you turn your gameboy on and your current game stats appear, you should see an opntion called "Mystery Gift". If you don't, it's because you haven't talked yet to the little girl in Goldenrod City's Departament Store. When the option appears, select it and press "A". You should see this screen:

Now you have to get two gameboys together (both gameboys' infrared ports must be facing each other) and press "B". Both players should get a gift. The gift will be cancelled if you didn't retrieve your last gift from the Pokémon Center. In order to do this, enter any Pokémon Center and go up to the second floor. A man dressed in blue will be waiting for you. You can get only one item per person, per day.

By using the Mystery Gift, you can get all sorts of cool items to decorate your bedroom, such as dolls, posters, carpets and beds.

This table lists all the items you can get, and how rare they are.