After a round trip of one hundred years' planetary time, the voyagers return ten years older. This lets the author maintain some continuity. The other approach is to use generation ships. People are born, live their entire lives in the ship, and die en route. Their descendants will be the ones to reach the destination.
Generation ships are plausible, but limit the stories an author can tell.
All the action takes place in a fixed environment. While relativistic STL ships are at least superficially plausible, they suffer from serious practical problems. The primary objection to relativistic travel is due to its speed. Say you're in a ship traveling at .999c. At this constant velocity, after a round trip that took 100 years of planetary time, the passengers would be about 4.5 years older.