Activities are a great way to engage students with Mathematics. And that also helps them to visualise mathematical facts. For example, Pi in its decimal form appears to have no pattern (we used to say). But, if we ask students to colour code digits (upto some 500 or 600), they will automatically be able to see that.
If we allow students to play with periodicity of repeating decimals after a while they will figure out Midy's property. https://web.iitd.ac.in/~atripath/publications/Midy.pdf
So, what all other concepts can help students to get engaged while having fun.