The idea of rather simple: take most of the planets in the solar system, chew them up, and then turn them into a ring as long as Earth's orbit, as wide as the planet, with 1000 mile high edges to keep the air in. A Ringworld would certainly give you lots of extra space – something on the order of three million earths – and, like Globus Cassus, it would be spun to make fake gravity. You could even make parts of it higher off the surface if you like your air a bit thinner, and if missed days and nights then you could put a row of black squares in an inner orbit to cast shadows.
This is just one of the megastructures you'll see at Dark Roasted Blend. Link
(Image credit: Stephan Martiniere)