Animating the aliens would require updating the Entity class to support multiple sprites. Then depending on the time passed the sprite being drawn for the entity could be swapped.

Make it possible to pause the game by pressing a key (P?). This should pause all game updates but still continue to render the screen and the entities within the game. It might be nice to add a message indicating the game is paused.

With the addition of the a new Entity the aliens could be made to fire back. This could either be done by timing it so each alien fires back every so many seconds. Alternatively, this could be based on a random occurance, say the alien has a 1 in a 100 chance of firing back at any given movement.

Add a frame rate counter to the screen while the game is playing. This could be done by counting game loops and recording the number counted each second. This value could then be displayed as part of the drawing section of the game loop.