lego, technic lego (they don't really make the stuff anymore) and origami. I used to make little boats with walnut shells and toothpicker woods, and tons of glue.

Also there were these foam puzzles of 6 pieces which fitted inside a frame, or you could make a cube from them. I also remember playing a lot with lego.

I also kept a mineral collection (crystals only) which I still have, but forgot the names of most of the stones (damn). And a lot of lego-ing.