Hard mode:

Pause the game and press Select. This will make the preview window disappear when you resume play.

Screen savers:

Turn the game on and let demo mode start. Press Select to bring up a swimming fish screen. Press A to return to demo mode, then press A again to bring up confetti and the Nintendo logo. Press B to return to demo mode, then press B again. You will eventually see a chalkboard screen saver.

Fireworks Screen Saver:

Beat a high score.

Move blocks up:

While any piece except a straight bar or block is descending, hold Left or Right until it is against the wall. Now while still holding Left or Right, rapidly press A and the piece will start moving back up.