Israel’s OffBits has produced “build it yourself” kits, comprised of throwaway screws, springs etc. Kids can assemble the kits into cool little toy robots and add their own “bits”.
http://www.israel21c.org/headlines/kits-to-design-toy-robots-from-spare-parts/
https://www.kickstarter.com/projects/theoffbits/the-offbits-the-art-of-spare-parts