Paddington Bear!
Some of the Enid Blyton mystery series, like Famous Five.
Is Geronimo Stilton too young?
I second Paddington Bear. My son (6.5) still loves him.
This summer he's discovered Geronimo Stilton and is reading this on his own...coming to me for special words he doesn't understand. For instance, today it was the word, "Stout".
I went online and we laughed over the audio of "stout"....(stout, stout, stout, stout, stout...ad infinitum)
But he got the meaning of a word that isn't used much.

By the way, my son has books from Dr. Seuss that he still wants to come back to. Usually at night when he wants us to read to him so he can just "chill".