Every time I think I have seen every toy, I come across something like the Dado Squares that catch my eye. These cool puzzles turn into anything your child can imagine. An airplane, skyscraper, blocks, whatever, turning flat colorful pieces to life to make play time fun.
I had a set of squares just like this (except they were translucent) when I was 5-- in fact I liked them so much they stayed around- I just unearthed a few stragglers in a box of toys I passed on to my own child. I'm psyched that I can expand the set with these! They're great building toys and they're nice and portable, too.