We've had some success with toy microscopes and simple experiment kits. We got a beginner's microscope set that came with some reagents and slides, which kept them entertained for hours. They're still young so the sturdiness of the pieces and the simplicity of use was important; it needed to be fairly robust and straightforward, which this definitely was.
We also enjoyed some simple chemistry experiment kits - again, designed for young children, with non-toxic reagents and easy-to-follow instructions. It's amazing how many cool things you can do with baking soda and vinegar! We've also branched out into magnet toys, which help teach them about polarity and some basic physics concepts in a fun way.
Our toy box is also stocked up with classic toys like building blocks, nesting dolls, and good old LEGO - which never goes astray! I agree that the simpler, the better; especially when they're young. I'm keen to instil that sense of wonder and curiosity too, and love hearing their wild theories and observations!
What other toys or books have you found that spark their imagination? Always good to swap notes!