Actually, you guessed it when you mentioned Harry Potter. :) Diana Wynne Jones is a British fantasy novelist who was writing better stuff more than a *decade* before Harry Potter was born. :)
I'm always mentioning Diana Wynne Jones to young readers as an alternative to Harry Potter.
Start with her wonderful book *Dogsbody* if you can find it. It's very touching -- as well as wonderfully imaginative.
And, of course, I'll bet you've read all of C.S. Lewis, too, right? Another great alternative to Harry Potter. Not to mention Alexander Lloyd's novels.
It's not that I'm down on Harry Potter -- those books are entertaining as well. It's just that there is so much BETTER stuff out there that never got all the publicity that the Potter books have gotten.