++ No matter the age, telling Math history could be useful.

Some stories are just fun and could change the mood in a classroom! Did you knew Évariste Galois, one of the most influential mathematicians of all time, died very young in a duel? What about Leibniz–Newton calculus controversy?

But more importantly some of these stories could help to understands math problems better. Why people tried to solve this particular problem? And how did they manage to solve it?

++ Another useful thing would be this: making problems less abstract! Telling students about why this formula is important? Which physics problem it can solve? How could be related to real life? etc.

++ Fun math problems could be useful too. Raymond Smullyan books are an example. But I'm sure there are lots of other books...

Do you know any online resources for this?

Vedic Maths

Dropping some links:

https://www.iiva.in/

https://www.vedicmaths.org/

https://vedicmathschool.org/

Lower student to teacher ratio. That way you'll get faster feedback as to what's not working in any particular case. Let the student tell you what they need.

Practical examples, like word problems, but concrete.

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Or ask them what they want to know and work it out with them, self directed learning.