Provide Lego. Lots of it. Being able to think of an idea, try making it, see results, assess the results, and alter it to make it better is a real benefit for anyone going into STEM programs.

A cheaper alternative (if you already have a computer) is MineCraft. It's like a virtual Lego set, but with creatures added.

And time to just putter with Lego or Minecraft. No structured activities or tasks, just time to say, "I think I'll build a castle", then do it, then modify and alter it repeatedly.