One of the possible results for the SC3 world generation tables is a result of "really physically bizarre". This was the result rolled by Klaxon, my son, when creating one of the playtest settings for SC3. He thought about it, and came up with "the River".
The River is the world partially natural and partially constructed. The natural part is a gas Torus in orbit around a neutron star which is itself in orbit around a normal star in its Goldilocks zone. The gas torus is about 1,000,000 km thick, and circles the neutron star outside its Roche limit a very fast orbital speed. This gas is apparently the result of a gas giant being pulled apart by tidal forces and gravity. The constructed part was found in situ when the world and the system or colonized. This consists of a variety of asteroids – apparently an entire asteroid belt – being pushed within the glass torus, and the eponymous river.
The river is a kilometers wide ribbon of water which meanders throughout the dense central ring of the torus. It is constructed – apparently by the Seeders – with gravity nodes which propel the water of the river along all the way around the torus. It waters many of the asteroids in its path, and can be sailed by watercraft throughout its length.
The central part of the gas torus is much thicker, and is at a breathable pressure. There is life throughout the torus, which gives it an oxygen atmosphere. There are settlements on the asteroids as well as constructed in open air. The asteroids have microgravity, and the river has a heftier artificial gravity, but the rest of the world is gravity free, with only constructed gravity available. The thickness of the atmosphere shields the river from much solar radiation.
This setting can be used in any science fiction game which has FTL travel and gravity manipulation.