Interesting Topics to Teach a Kid?

The other answers are good, especially the one about getting out and about, but the kid is seven, and as you say, smart. Why not ask him? He may have some interests that you could investigate.
He'll start to think a lot more if he's not spoon fed. Think also about hands-on things you could get into. Cooking, building stuff (like a go-kart or a bird feeder) making inventions, etc. You two could research the project, go out and
buy the materials for it, and then create something. You both might learn things.

When I was seven I had different interests than most kids.

Witches, ghosts, serial killers and art. No joke.

I agree with the person who suggested you ask what he would like to learn.