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'll 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's against the wall. Now while still holding Left or Right, rapidly press A and the piece will start moving back up.

Special thanks to Revolution reader Bryan Lawrence!