I will second The Chronicles of Narnia: in terms of children's novels that are both light-hearted and complex, you can't get much better.
I'm surprised that I haven't seen more people recommending The Hobbit, though - it's one of the best children's books of all time! Not least because it remains entertaining no matter what reading level or age you are. So I highly recommend that one.
Now for two unorthodox options. First, I recently started reading a classic in the adventure genre, one of the first real swashbuckling tales of exotic lands and simple fun, and that is King Solomon's Mines. Despite being a very old book, it is written in a very simple and straightforward way that wouldn't pose a challenge to a smart kid like your goddaughter, and was actually a childhood inspiration to a whole generation of adventurous writers and leaders: C.S. Lewis, J.R.R. Tolkien, and Theodore Roosevelt were all huge fans of it growing up, and some of it inspired their own works.
I will say, however, that it is a product of its time: being set in colonial Africa lends it to some (relatively mild) racist content - there are no n-words (aside from a little one in the introduction) but there is a subtext behind it that you may want to be careful of. I would still say to try it, though: perhaps read it yourself to judge whether its appropriate or not, but most kids would latch onto the adventure aspects and pass right over the questionable stuff, and again its very mild anyways.
And for my second recommendation, I would consider trying an early sci-fi novel: A Canticle for Leibowitz. The basic premise is monks trying to preserve knowledge and learning after a nuclear war wipes out civilization and fosters violent anti-intellectualism in the remaining humanity. It isn't very long, but it is dense and tightly packed with symbolism and complex ideas. the actual content of it would be completely age appropriate - there isn't really any swearing or gore of any kind - but its weighty enough that I would only recommend it if you feel she would be able to grasp it all. I'm sure you're a good judge of that, though, so take that however you will.