The point of the exercises in my current class is to keenly observe the minute details present in natural objects. Sand dollars are all different in size, shape and color, and the markings are teensy little hatch marks. It has been a nice change to travel light to class (no paints other than watercolor or pencils) and to slow down and really observe and appreciate.
Sunsets are beautiful to watch, but I think very difficult to paint. You just can't improve on something so beautiful. Our family recently spent several days on Captiva, Florida, where the sunsets are magnificent. People line the beaches just to see each evening's performance. It is reported that some see a bright green speck just before the sun sets, but unfortunately we did not see it.
http://www.dailypaintworks.com/fineart/diane-campion/501-captiva-sunset/290524