Daily training
Earn stamps by playing through one of the recommended training activities every day. Stamps will unlock the following bonuses up to a certain point, after which you'll just unlock hard modes for all the activities you haven't already done so for.
Get more games
Here is a good way to unlock more activities in a single day, rather than waiting for them to unlock over time. All you have to do is turn the game on, do a given activity, shut the game off, remove the game from the DS, then put the date up one day. Put the game back in the DS, unlock and play the next activity, and repeat the process as needed.
Hard modes
Attain the highest ranking for an activity to unlock its hard mode. Core games require an "Advanced" (80%) and sports games need a "Pro" (90%). Alternatively, you can unlock the hard modes by earning stamps after all the activities have been unlocked.
Special messages
Play the game on various celebration days (Christmas, New Year's, Halloween, Birthday etc.) to receive a warm welcome when you begin.
Daily training...