I'm with bzylzy. I can't speak for most kids, but mine both thrive on doing "grown up" things. They fight me on their chores every day, but just something different from the routine often gets them thinking again. This weekend we are planning on planting vegetables - I have yet to be able to keep a plant alive, but they are looking forward to it, and it can be a learning experience for them. Carefully selected extracurricular activities may help as well. My son takes violin lessons, and while he has a talent for it, his teacher always makes a point to give him something to work on that is somewhat difficult for him. He gets a huge thrill from working hard at something and finally mastering it (as opposed to assignments at school, which he finishes quickly and easily). I also try to challenge them in normal daily activities. We went hiking last weekend, and I showed them what quartz looked like - they both started hunting for pieces of it along the trail. That led to questions about why some rocks were smooth and others were jagged - which led to discussions about places where water has obviously been running recently, why it was dry now, and if you were lost in the woods and needed to find water, what would be the best way to find it?
I'm struggling with finding ways to challenge both of mine and keep them interested as well, so I don't know how effective this will be for your child. But it works (mostly) for mine, so hopefully I'm not the only one!