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