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.
Effect
How to unlock
'C' Count
Get 7 stamps
Basketball
Get 4 stamps
Circle Spot
Get 5 stamps
Custom Training mode
Get 1 stamp
Eye Knowledge mode
Get 14 stamps
Fast Match
Get 9 stamps
Football
Get 10 stamps
Letter Count
Get 3 stamps
Number Tap
Get 11 stamps
Record Challenge mode
Get 13 stamps
Soccer
Get 8 stamps
Stamp Design no.5
Get 12 stamps
Stamp Design no.6
Get 15 stamps
Table Tennis
Get 2 stamps
Volleyball
Get 6 stamps
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...