[Irish Wolf]

Which of Niven's settings? The Ringworld spins for effective gravity - its rotational speed of 770 m/s gives it slightly under one Earth gravity. The air is held aboard by massive walls around the edges.

The Smoke Ring is in fact a ring of gases, with low gravity throughout, spun off of a gas giant orbiting a binary pair where one star is a yellow dwarf and the other is a neutron star. Maneuvering through the Ring is... tricky; most people never leave the Integral Trees they grew up on.

The disc he conceived (but never wrote stories on) is a flat disc like a record, with a star in the middle, and air once again held in place by walls. Gravity would be perpendicular to the plane of the disc, and the area would be in a perpetual twilight (although a day/night cycle could be approximated by "bobbing" the disc "up" and "down").

However, if you decide to solve any issues using magic, make sure you don't also use his magic system. You wouldn't want to be living in, say, the sphere discussed earlier, with things kept from falling into the sun with magic, when the mana fails...