Each day below runs in the order the stops are visited, with one substitution worth making when it rains or when your legs are spent. What matters is the sequence and where the gaps for meals fall.
Day 1: Gyeongju, Museum Then Tombs
Day 1 starts with the transfer from Seoul, so treat the drive as part of the day rather than something that happens before it. Pickup is at your Seoul hotel, and Gyeongju National Museum opens the day because it sets up the three sites after it: the pond, the burial mounds and the observatory.
The afternoon belongs to Donggung Palace and Wolji Pond, then Tumuli Park, then Cheomseongdae Observatory, which closes the day. The park and the museum are the two long blocks at an hour and a half each; the park is the one where tired legs show, because it comes last in the afternoon.
The night is in Gyeongju, dinner is on your own, and breakfast the next morning comes with the room.
If it rains, cut the observatory and give the museum the half hour you save. Of the four stops it is the shortest and the easiest to lose without breaking the day.
Day 2: Two Temples, Then Busan
Seokguram Grotto comes before Bulguksa Temple, and the order is deliberate. The temple is the morning's main block, so the grotto visit stays short and the car turns toward Busan with the afternoon intact.
That afternoon is the downtown cluster first: Yongdusan Park, BIFF Square and Jagalchi Market, then the Oryukdo Skywalk on the coast. Yongdusan sits in the quarter where a Japanese trading settlement once stood, and the three central stops run back to back, which is why the afternoon holds four of them.

Jagalchi does double duty as the meal stop. A market is the simplest place to eat without losing the afternoon, so eat properly here and keep the evening light.
The night that follows is in Busan. The skywalk is the last stop of the day, so dinner lands wherever you end up.
Day 2 is the longest day of the three, so city shoes work better than boots.
In rain, keep both temples and be ready to drop the skywalk, since the market and BIFF Square are the stops that work under a roof.
Day 3: Temple, Beach, Village
Day 3 opens at Haedong Yonggungsa Temple, then moves to the Haeundae pair: APEC Naru Park and Haeundae Beach. The beach is the largest in the country, so the stop there is a walk on the sand rather than a swim.

Gamcheon Village closes the loop through the older lanes. The appeal is contrast: streets that carry the look of an older Busan, sitting in the same trip as the skyscrapers along the beachfront.
Breakfast comes with the room again, and lunch is your call between the beach and the village; after Gamcheon, the next meal is on the road.
Then comes the drive back to Seoul, and the tour ends in Seoul or near it, so keep that evening free.
The drop-off at the end mirrors the pickup at the start, so a Seoul hotel on both ends of the trip saves you a transfer with luggage at night.
If weather shuts a site, the operator swaps it for another or refunds that part of the package, which matters on a coast where wind can close a walkway. On a tired afternoon, the beach is the block to cut.