At first glance, the call to action would be to avoid being swallowed by a big fish! However, a close reading of the book of Jonah shows us that its call to action is to obey God—no matter what he asks us or how difficult it may seem.
Being swallowed by a fish was Jonah's God-given opportunity to take time and think about what he had done, after he had rejected God's call to travel to Nineveh and warn the people there that their wickedness would imminently cause the destruction of their city.
The Ninevites, however, were known as a vicious people, and Jonah, reacting in fear, rejected God's will. Deciding to flee Israel by ship, he boarded a vessel heading for Tarshish, but God brings up a storm, putting everyone on the boat in mortal peril. Jonah immediately realizes that he is being punished for disobeying God and, fearing for the lives of everyone on board, he instructs his crew mates to throw him overboard, which they eventually do.
The call to action of this book—to obey God no matter what—still hadn't quite sunk in for Jonah after a spell in a fish's belly, and God finishes teaching him the lesson by providing a vine and having it die after Jonah fails to take care of it.