https://neal.fun/deep-sea/ Experience how deep the ocean can go. May cause thalassophobia.
https://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html Scale the universe if the moon were the size of a pixel.
https://www.robinwords.com/ Fun word ladder game to test your vocabulary.
http://make-everything-ok.com/ For when you feel like things are just not going your way.
https://stellarium-web.org/ Look at and learn about the stars.