have you ever played "Bridge Builder?" Its real low budget in terms of graphics (at least way back in the original versions).
A nice iPhone port shouldn’t be too difficult…
Also, adding to the concept, what about building earthquake-proof skyscrapers in california (e.g. with sway)?
Or feck-off-sized bridges (like the one in France) that is literally in the clouds for miles, along with high winds, etc?
Building cathedrals is another one you could look at. saw a great show on the documentary channel (history’s worst jobs or something to that effect) and it showed how dangerous and insane cathedral building was in the 1500’s (or thereabouts).
-Brendan.