I know she's reading more complex books, but I remember trying to find all the Encylopedia Brown books. They are more for working your mind than necessarily for straight story, though -- trying to find the clues in the story to solve the mystery each short story presents. I don't know how popular they are now, 30-40 years later. "The Great Brain" series (about a young Mormon I think, growing up out west, and his older brother Tom was really smart and always out for a con) was fun and not a hard read, they can be plowed through pretty quickly.
I really liked LeGuin's Earthsea stuff. The first three books were around when I was young, but she later put out Tehanu, Tales from Earthsea, and I think The Other Wind. They might or might not be too much, but she's been able to handle Harry Potter fine. LeGuin's kind of a tricky writer, you can read her stuff easily enough but you might miss some of the reverberations of what she's written until you think more about it, maybe even later in life.
Robert Asprin's "Myth-adventures" series is also a lot of fun, for a fantasy series.
I know there's probably a ton of modern stuff I've never heard of, in the current YA publishing wave. I know The Hunger Games stuff is oversaturated in the public mind, but the books are actually a decent, thoughtful read, and Katniss is more nuanced and difficult than the film version.