Here are some challenges to make the game better. You don't have to try them in order. Do them in any order you want.
Challenge 1. Different Speed
Make the enemies move at different speed.
Challenge 2. Increasing Difficulty
Make the monsters appear more frequently as time passes. You can also make other changes to increase the difficulty of the game (eg. make the enemies move faster).
Challenge 3. Animate the Player and Enemies
Both the player and enemies have multiple images that you can use to create an animated effect. You can use the pgzhelper module from the Ninja Runner game to help with this.
Challenge 4. Ending Music
Stop the background music and play a different music when the game ends. I only provided one music file, so you'll have to find a suitable one on your own. The music file must be in either the .wav or .ogg format.