Ah, megascale engineering is fun! Of course, beanstalks and terraforming are already in this category. Some other ideas: Tent/blankets for Martian cities: rather than building rigid domes it is sometimes cheaper to use flexible transparent film held up by air pressure and/or bracing. The film can be single-walled or consist of internal airspaces with protective/buoyant gas. This approach can be extended to make "worldhouses": large stretches of landscape kept under cover. In principle one could build something like this around an entire planet, producing a glassed in world. http://arxiv.org/abs/0902.0656 http://arxiv.org/abs/physics/0701098 One can build fun alternatives to space elevators under some conditions. Lofstrom loops use streams of electromagnetically managed metal to launch things into space, and one can build towers in the same way. Bolonkin suggested using electromagnetic fields to do it. One can also use Eiffel-tower like tapering structures and tensegrities to make towers at least a hundre kilometres high in Earth gravity. https://en.wikipedia.org/wiki/Launch_loop http://arxiv.org/abs/0704.3466 I'm of course all in favour of building Jupiter Brains, planet-sized and upwards computational structures. The smallest would be diamond planets, the most powerful would be quasar-bright structures of nuclear matter (I was told yesterday by Hugo de Garis that he had found a theoretical way of doing nand-gates using gluons and quarks - yay!) powered by entire globular clusters. http://ftp.nada.kth.se/pub/home/asa/Work/Brains/Brains2/ Moving planets using Kuiper Belt objects steered onto carefully designed paths might be possible. First the object does a gravity assist with an inner planet, giving it some momentum. Then it does a gravity assist with an outer planet, taking some of the momentum. Repeat for a million years... or a stream of a million objects. http://arxiv.org/abs/astro-ph/0102126 Then there are all the nice things one can do with Dyson shells. Besides using them to collect all energy and run computation (Robert Bradbury's Matrioshka Brain), one can use the surface to make a phased array laser weapon able to vaporize planets several lightyears away (my own Death Star idea) or to turn the whole thing into an engine for moving the star. Partial shells are good for powering the planetary disassembly systems you need to get the material for the full shell. https://en.wikipedia.org/wiki/Matrioshka_brain https://en.wikipedia.org/wiki/Stellar_engine http://www.sciencedirect.com/science/article/pii/S0094576505003012 And finally, there is star lifting, mining entire stars to get material or ensuring that they will burn longer. https://en.wikipedia.org/wiki/Star_lifting