My Side of the Mountain. It was the class favorite of my 4th grade homeroom. We had the teacher reread it to us twice by popular demand. I think I read it a few years later when I was 14 or so, but I haven’t read it since and I will never want to.
I love short story anthologies for these sorts of things, like The Time Traveler’s Almanac. The time travel mechanic is almost always explained in every story since it usually sets up or drives the conflict in the story. You end up with back to back bites of stories that explore lots of ideas.
I no longer re-read books from my childhood and adolescence. Reality almost never lives up to the memories of reading it when I was a kid. It’s better to leave those things in the past where they remain meaningful.