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One last idea. An achievement for knocking of two cans with one ball when the cans are at least two can widths apart. Like picking up a split in...
You can have an achievement for beating the entire game with only 1 ball. One for hitting every special can (explosion, extra ball) in the game....
How about an achievement for knocking a can out from the bottom of the tower and having the can on top land/stay on the platform?
Rebound - Knock down a can after hitting the wall. Quick Draw - Hit a can that is in the air. Think Fast - Beat a level in under 3 seconds....