4 ideas
In Achille Eyraud's 1865 novel, a traveler reaches Venus in a vehicle that moves by forcibly ejecting a fluid out the back, not by balloon, cannon, or anti-gravity substance. It is usually cited as the first fiction to propel a spacecraft by reaction, the principle real rockets use, in the same year Verne fired his travelers from a giant gun.
A vehicle ejecting mass in one direction gains equal and opposite momentum, so propulsion works in empty space where there is no air or ground to push on. The key conceptual step is that the craft pushes against its own expelled material, not against the environment.
An early inventor or imaginer can identify the correct governing principle while getting the practical mechanism wrong. Eyraud's engine is often described as reusing its ejected fluid, which would undo the thrust it produces, yet the core insight of reaction propulsion was sound. Priority on a principle should be judged separately from whether the first implementation would actually work.
Sending a traveler to an alien society lets an author show familiar institutions from the outside. Customs that seem natural at home look arbitrary once set beside a plausible alternative. The destination works less as a place than as a controlled contrast that exposes the assumptions of the author's own society.