Practically speaking (running a little maths circle in the UK for my children, and some of their friends from nursery and primary school), I have found the Nrich website to be the single best source of resources:
https://nrich.maths.org/about-nrich
The book by Zvonkin described in the article is a very good motivator, particularly for the honest descriptions of lessons gone badly wrong, and staying up late cutting out pieces of cardboard! But it's quite difficult to use as a teaching resource.
There is also https://parallel.org.uk/ by Simon Singh, but this is aimed at ages ~10+.
The book by Rozhkovskaya has some really nice activities in it. https://www.amazon.co.uk/gp/product/1470416956
The book by Zvonkin described in the article is a very good motivator, particularly for the honest descriptions of lessons gone badly wrong, and staying up late cutting out pieces of cardboard! But it's quite difficult to use as a teaching resource.