Building kit for open-ended play

It is wonderful inspire children to question "What is it?" and "What does it do?" "What can I build?" I want my designs never to be specific as dolls or cars and I want playing objects to encourage exploratory, unstructured play. I want them to interact with things that are well made, well designed, and show them that they are worthy of solid, "real" things. I want to inspire children with the design of an open-ended play.