TED-Ed, I Said...
In past issues on this website, I have recommended TED videos. Now, I also recommend TED-Ed, a refreshing blue sky amidst an on-line world of low-quality lesson plans.
Though it's options are growing,
TED-Ed offers around 100 videos. With its "flip this video" option, you can turn a video into a customized lesson that can be assigned to students or shared more widely, complete with a Quick Quiz, a Think opportunity, and options for Digging Deeper (often with additional links). You can add context, questions and follow-up suggestions.
To introduce you to the mathematics side of TED-Ed, I suggest you start with these videos:
Symmetry, Reality's Riddle
One is One...Or Is It?
How Stats Fool Juries
How Algorithms Shape Our World
The Beauty of Data Visualization
The Best Stats You've Ever Seen (Has been recommended earlier, but if you missed it, now is the time to see it!
The Beautiful Math of Coral
Many more video experiences are available, though not directly related to math (i.e. they might possibly still have value!).