Collect all three pink Elebits. To capture the third Pink Elebit, do the following.
You can unlock extra modes for every non-boss stage!
In order to unlock a level in level editor, you must rank a C or higher in it. If you rank a B or higher, you get a set of items corresponding to that level that can be used in any place.
After defeating the third boss, find and collect the rare item in certain levels to unlock special abilities.