For the first challenge, add the ability to reverse the sunset after it is completed, so your user can press for a sunset, and then press a second time to get a sunrise. You will need to build another AnimatorSet to do this – AnimatorSets cannot be run in reverse.
For a second challenge, add a continuing animation to the sun. Make it pulsate with heat, or give it a spinning halo of rays. (You can use the setRepeatCount(Int) function on ObjectAnimator to make your animation repeat itself.)
Another good challenge would be to have a reflection of the sun in the water.
Your final challenge is to add the ability to press to reverse the sunset scene while it is still happening. So if your user presses the scene while the sun ...