Ruby And Sapphire: Secret Base
The "Himitsu Kichi" or Secret Base is a place a player can set up a little house type deal. You can set up your base in a small mountain (left) In this secret base it has been said that you you can let your pokemon free to play around in the secret base, this has not been confirmed, but by the looks of the screenshots, you can at least set a few free to run around out of their pokeballs.
You can probably recieve certain items throughout the game, such as posters or dolls, that you can decorate you base (similar to your room in G/S). You can also trade secret base designs with your friends via link cable.