Busan is the better day-trip base of Korea's two big cities, for one plain reason: the good options are closer.
Gyeongju is the one to do
About an hour by train. The historic core — royal tombs, the observatory, the café street — is walkable once you arrive, so the day is spent looking rather than moving.
Bulguksa and Seokguram are up the mountain and are a separate half-day, so a single day trip covers either the centre or the temples comfortably, not both at leisure.
Tongyeong and the south coast, if you want scenery
Further out and most of a day, but it is a genuinely different landscape — a cable car over the harbour, ferries to nearby islands, and a working port town rather than a historic one.
Right for a second day trip, or a first one if history is not the draw.
What is not a day trip, despite the lists
Haedong Yonggungsa, the cliffside temple, is inside Busan. It is a half-day at most, best paired with the beach or the aquarium since you are already out that way.
Gamcheon Culture Village is also inside the city. Both get listed as day trips, and planning a full day around either wastes a morning.
Seoul is possible and usually pointless
About two and a quarter hours each way on the KTX. That is four and a half hours of travel for a few hours in the city. Worth it only for a fixed event.
What is checked here and what is not
Relative distances and what each place is are stable and checked on the date above. Train and ferry timetables change — confirm the current schedule before planning a tight day.
