Check out these sites:
https://www.hoagiesgifted.org/links.htm
https://www.factmonster.com/
www.chesskid.com
www.codeacademy.com
www.nasa.gov
www.natgeokids.com
www.envisionexperience.com
(create and run your own math scavenger hunt!)
www.bestrobotever.com
(pants on fire podcast)
www.nga.gov
(National Gallery of Art)
www.readingrockets.org
www.bookopolis.com
www.journalbuddies.com