Lego and origami
Those were two of my favourite things as a kid. They still are. Maybe they helped, both my brother and I were pretty good in maths and science. OK, a sample size of two is not that significant, I admit :D
Still, time to get some paper and teach a kid how to fold (the traditional goldfish is great, very forgiving, looks good when glued onto blue paper a well!). Then, there's a ton of simple modular flowers, boxes, boats... (and the steam train, which I still struggle with).