Shoot Targets.
One tip I can offer is add an event where when player clicks on "Play again" it sets the timer back to its original setting. In this stage of your game it will give you a negative score because the timer doesn't reset.
Also a few really simple changes can make this game way better:
1. upgrade sprites from MS paint maybe (you could use a more advanced program like PS or GIMP and import them)
2. extra obstacles, like moving targets or spikes that hurt you
3. sounds and music
4. more feedback (maybe when targets get shot they explode or something)