Change the Image After Every X Taps
That allows the user to progress to a new image after they have successfully tapped X number of times which you can specify. Imagine breaking a cookie apart to reveal a message or load a bottle with Co2 to make it explode.
Awards for All Finishers
Set prizes to all players who finish the game successfully or reveal the prize code after tapping enough times.
Leaderboard
Place all of the players into a leaderboard to see who scored highest in this very competitive game type.