Erector / Meccano is a nice open ended STEM toy that could use some disruption and metrication and modernization and no themed disney licenses please.
Unfortunately there is this strange attractor that sucks everyone who trys it into "I wanna make kids version of 80/20 because 80/20 is both cool and insanely expensive" leading to "heres my custom aluminum extrusion of immense expense that works with nothing out there, but now I know why 80/20 is incredibly expensive" leading to "remember those guys?"
I personally think MakeBlock (http://www.makeblock.com/) does a great job of being a modernized metricized replacement for Erector sets.
It's not particularly cheap - probably costs ~$300 to get a decent sized kit or a smaller kit with enough spare parts to experiment.
All of the mechanical parts are machined reasonably well and fit together in a large number of useful configurations - I really like them for rapid mechanical prototyping.
The electronics are basically arduinos with beginner-friendly connectors - haven't used them myself but have generally heard good things about them.
One of the things I have found frustrating trying to get Erector/Meccano for a new generation is that I can't seem to find anything that's just a big bucket of parts. The generally available things seem to only be kits, and those which are a bucket of parts are woefully lacking in fun bits.
Unfortunately there is this strange attractor that sucks everyone who trys it into "I wanna make kids version of 80/20 because 80/20 is both cool and insanely expensive" leading to "heres my custom aluminum extrusion of immense expense that works with nothing out there, but now I know why 80/20 is incredibly expensive" leading to "remember those guys?"