To enable level select, go to the Passwords screen and enter this code: NTTTT.
Unlock Hidden Art
To unlock hidden concept art, enter MFKRPH at the Password screen.
Level Passwords
At the Password screen, enter these codes to unlock different stages:
2: PBBQBB
3: QBCQBB
4: SBFQBB
5: DBKQBB
6: NBTQBB
7: PCTQBB
8: RFTQBB
9: CKTQBB
10: MTTQBB