Things to try:

1. muck about with the colours. Set a background colour or even a background sprite
2. figure out how to make levels(maybe new html page for each level)
3. levels with specific brick placements rather than generated from for loop
4. indestructable or multihit blocks
5. replace paddle, blocks and ball with image sprites. CHeck code for things which depend on h/w attributes of previously drawn shapes and check they're still compatible with img call
6. tart up page wth css
7. sounds for wall bounce, paddlebounce and block destruction(freesound.org)
8. background music(incompitech.com)