I hope you enjoy your re-read as an adult! As for other children's books, I really love the Arthur Ransome series, Swallows and Amazons. They are nothing like The Phantom Tollbooth but great fun if you are looking for other recommendations!

Aw! I loved the Walker kids and their adventures!

What about the Finn Family Moomintroll series? Are you into cute little white Hippos? ;-)