There are two outside areas - one to the left of the house, and one to the right of the house. When you go outside, you usually go to the area on the left (where you can plant a tree.) You can see the area on the right only during some seasonal animations (such as the Sapporo Snow Festival in February, Cherry Blossoms in early April, the Hydrangea Festival in June.)
It's been a long time since I ran a TMGC+C, and I don't remember exactly how to get to the these seasonal animations.
They may be related to the screen saver: If you don't touch any buttons for a while, the backlighting turns off. Wait another 5 minutes or so, then press a button. The backlighting should come back on, and if it's the right time of year, you may see an outdoor seasonal animation.
Or: perhaps the seasonal animation may appear if you simply send your character outside.
Sorry. I don't remember.
If you happen to see one of these season animations, can you let us know. (So we can look it up next time someone asks.)