[@PerfectThought]

Solar sails are interesting, though I think might be less useful the farther you get from a star, though im honestly not an expert on such things, and Im not sure how much thrust you get out of it.

Theres also a type of proposed engine called the "Medusa Drive" https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion#Medusa Which is like a solar sail but you throw (low yeild) nukes in its direction. its a bit more complicated than that, but its a pretty promising hard scifi design.

As for FTL drives, I think something like dune would be a fun way to do it, especially the depiction in the new movie, where it is unclear if the ship moved or is simply two places at once