Blocks

Rachel Whiteread kind of uses building blocks in her work ... although they are all ready set out so not as interactive it is still an example of using blocks to build and balance things. Tate Modern's vast Turbine Hall,

It is more interactuive as people go around witnessing the massive scale and maze like layout of the work.

"Whiteread's choice of subject-matter reflects an awareness of the intrinsically human-scaled design of the objects with which we surround ourselves and exploits the severing of this connection, by removal of the object's function, to express absence and loss. Her early work allowed autobiographical elements. Later works move towards the expression of a universal human position, and their titles become correspondingly more prosaic."
http://www.tate.org.uk/servlet/ArtistWorks?cgroupid=999999961&artistid=2319&tabview=bio

Lego

Taking lego towers to the extreame - another pitch idea could be to buy a sugar load of lego and let people make their own bit then put it together at the end... much like a totom pole:

The contribution could be something important to the person but it doesnt have to be ... but if it was then maybe the art work would have more meaning to it .... but the prime reason to do this in the first place would be to see how tall it could go with out falling over.... balance related.