for a "O'Neill Cylinder"
one was in A. C. Clarke's Rendezvous with RAMA
https://www.youtube.com/watch?v=UiO8CgGiWPM

one can start with a metal rich asteroid
place very big solar mirrors around it to heat it
then SPIN it
it melts - add nitrogen gas to "blow it up" like a balloon
it forms a big long tube


aerostats in the Venusian skies
they are a bit easy

think of a "flying saucer "
or
Star wars and Cloud City where "lando calrissian" was
http://uncovermichigan.com/content/2...-possible-nasa