Give a kid a copy of...say, the “Narnia” series. Sword fights, action-packed battles, betrayal, redemption....what more could a young imagination want?
(Added, naturally, with good, healthy parenting and real-life activities.)
Or Tom Sawyer, Huck Finn, Treasure Island, 20,000 Leagues Under the Sea, Kidnapped, etc. I read them all multiple times when I was a child. Science fiction has hundreds of well written, action packed books.