Describe the theme.

http://www.gradesaver.com/rip-van-winkle/study-guide/major-themes/

The theme is the main idea or what one can learn from a story. In this case, you might say that "Helping others might gain you great benefit, including missing disastrous events of life (such as war)." On the other hand, you might say, "Sleeping one's life away is not much different from being idle when one is awake."

The themes found in Rip Van Winkle are all detailed in the above link. There are a variety of themes, if your teacher is only requiring one, you can take your pick.