I’m not sure if you’re looking for instructional material or fiction that also exposes them to other things.
For educational content, I recommend the Core Knowledge series (What Your ______ Grader Needs to Know) by E. D. Hirsch
Fiction that also informs:
Magic Treehouse
Time Warp Trio
American Girl
Magic School Bus (picture and chapter books)
Einstein Anderson
Sir Cumference
Really, any book can offer the chance to learn something new. Nancy Drew introduced me to a wide variety of subjects.
I agree with PP that screens today offer marvelous sources for learning. PBS generally has some great educational shows. YouTube can provide videos on any subject.
Hoagies has great links for enrichment for all subjects and levels:
https://www.hoagiesgifted.org/links.htm
There are websites like Scratch and Duolingo to learn about subjects like coding or foreign languages.
Games and aps offer fantastic opportunities for a child to learn and enjoy themselves in the process.