Kid's books

[ChristyHC]

My 8 year old ds is getting into reading. He is reading around a grade 3-4 level. He's been doing all of the Magic Tree House books, but I'm just getting sick of them and they are really not challenging him anymore. I'm looking for good suggestions for what to offer him to read this summer.

Also, do any of you remember reading a book series about kids who go on mystery hunts through the woods? I think it was while they visited their grandpa or something. I thought it was called Keys in the Wood, but when I search for that title I don't find anything.