Anyone remember Fischertechnik? Twenty years ago it was like scientific LEGO made with aluminium extrusions. No-one ever played with it; it was for building robots and 3-D printers and things. It's gone a bit plasticky since then.
Now a Dutch designer has scaled up the components, added pedals and wheels and things, and now you can design and build for your children their own bicycle, tricycle, recumbent trike, chopper trike, a Like-a-Skidoo, and probably a dozen other cool things!
If there's ever a miniArellcat, this is what they'll get for Christmas.