At the title screen, press Start to go to the title menu. From there, you can press A and/or B varying numbers of times (per the below list) for various cheats. The order of the button presses (other than Start) does not matter, but the quantities do.

Effect	Code
Level 2	Press A 6 times and press B 6 times, then Start.
Level 3	Press A 10 times and press B 9 times, then Start.
Level 4	Press A 13 times and press B 3 times, then Start.
View Extra Day 1 End Event	Press A 4 times and press B 4 times, then Start.
View Extra Day 2 End Event	Press A 6 times and press B 2 times, then Start.
View Extra Day 3 End Event	Press A 8 times and press B 2 times, then Start.