If Watson's portrait of Holmes is accurate, what is Holmes planning to do as the story ends?

At the end of "The Red-Headed League," Sherlock Holmes responds to Dr. Watson's enthusiastic praise and congratulations with these words:

“It saved me from ennui,” he answered, yawning. “Alas! I already feel it closing in upon me. My life is spent in one long effort to escape from the commonplaces of existence. These little problems help me to do so.”

Watson senses that his friend is going to indulge in his destructive habit of using cocaine...

At the end of "The Red-Headed League," Sherlock Holmes responds to Dr. Watson's enthusiastic praise and congratulations with these words:

“It saved me from ennui,” he answered, yawning. “Alas! I already feel it closing in upon me. My life is spent in one long effort to escape from the commonplaces of existence. These little problems help me to do so.”

Watson senses that his friend is going to indulge in his destructive habit of using cocaine or morphine to ease his ennui. These drugs were not illegal in Victorian times and little was known about their dangers. Two great names in British literature are associated with drugs which have since been outlawed. These writers were William Taylor Coleridge and Thomas De Quincey. Coleridge, the great poet, essayist, and philosopher, became addicted to opium, as did the essayist and critic De Quincey, who wrote a book about his experiences with the powerful drug titled Confessions of an English Opium Eater. Coleridge's best-known work is "The Rime of the Ancient Mariner." Holmes' passionate interest in violin concerts is probably another way in which the detective finds escape from the non-stop activity of his powerful mind.

Watson's quotation of Holmes shows how easily the great detective can become bored. In just one day Sherlock Holmes has captured one of England's most wanted criminals and saved an important bank 30,000 pounds worth of French gold. But once he has achieved this stunning victory, he quickly loses interest in it. He does not care about fame anymore. No doubt he will let the Scotland Yard man Peter Jones take credit for arresting John Clay and saving the £30,000 in gold.

When Holmes tells Watson that "these little problems" help him to escape from ennui, this is partially to explain why he took on such an apparently trivial case and why he takes on other such cases out of pure ennui. This desire of Holmes' to keep his mind occupied enabled his creator, Doyle, to invent many unusual plots and to have Holmes work for many people who might otherwise not have been able to pay him his usual high fees. Helen Stoner in "The Adventure of the Speckled Band" is just one example of a client who needs help but cannot afford to pay him.