Harder game
Right before you start a level, at the Street and Goal screen, press Select to increase your goal to 100,000G.
Outer Space board
To unlock the last board, you must first play and defeat the computer at boards (streets) 2, 3, and 4, in this order. Once the Outer Space board is unlocked, you may use it either for playing against other people or for single play. If you defeat the computer during single play, the game ending and staff credits will be shown.
Sound test
At title screen, press and hold Up & Select and then press Start. If done right, the sound test should appear.
Turbo file operations
At title screen, press and hold A and then press Start. If done right, you should enter the Turbo file manager.
Harder game...