Some enhancements our students added, in roughly-increasing level of difficulty:

Gradual speed-up
Splash screen	
Game-over message
Initial screen	
Simple scoring	
Quit	
Level-based speed-up and other features
New game
Outline pieces
Basic music
Sound effects
Advanced scoring
Saving high scores
Highlight lines cleared
Additional keyboard controls
Color cycling
Piece preview
Pause
Danger warning
Piece holding