Hello,
If I understand correctly, the car is convenient and inexpensive compared to the train.
But the idea would be to rent it as soon as we leave Seoul since I imagine it's better to use the subway in the city.
Yes, exactly.
However, if we go to Jeju, that’s where things seem to get complicated with the car rental...
So, we’d have to return it in Busan, pay an extra fee, and rent another car in Jeju if I understand correctly.
Can you confirm this? Or do you have other experiences?
Yes. Make sure to confirm if there are drop-off fees and how much they are.
After Jeju, we wouldn’t have a car to get to Suwon, but I imagine we can easily get there from Seoul’s arrival airport.
There are two airports in Seoul. Flights from Jeju land at Gimpo Airport, which is closer to the city center.
Another question that comes to mind: parking.
With a car, is it easy to find parking in cities and at hotels? Are they paid?
Yes, no problem parking at hotels, and I’ve never paid for it. You just need to select hotels that have parking. You might even discover the automatic parking system, like I often did.

Automated Parking Systems in Korea Explained | TikTok
As for hikes. If we want to include one in a park with beautiful temples and autumn colors, which one do you recommend?
It’s not in your initial itinerary. I really like Seoraksan Park in the northeast and Jirisan Park further south.
I’d read about Seoraksan Park, but it seemed too far for a day trip from Seoul if we don’t have a car at that point, right?
By car, it’s 2.5 to 3 hours minimum each way, depending on Seoul traffic. This park deserves at least a full day, ideally 2 or even 3 if you hike a lot. There’s plenty to do and see in the area, like stopping at Nami Island, Sokcho near the park, the DMZ in Goseong (my profile photo), and then following the east coast.
Next, about the idea of seeing the North Korean border—same thing, to do something “non-touristy,” I understood we’d need to go further north, so having a car. I’m still not sure about the interest in this excursion, to be honest... Especially with 4 days in Seoul. If I take out 1 day for hiking and half a day to see the border, there won’t be much time left to visit the city...
What do you think?
The most interesting DMZ is in the northeast at Goseong, but you need a car to get there. There are a few formalities to complete before arriving at the checkpoint, where the tension was very palpable when I was there. Very strict control of the completed papers, authorizations, passports, and your mobile phone, where you receive an SMS in case of an alert.
DMZ 박물관

Permit to place on the dashboard.

Your initial plan is great for a first trip given the time you have. Unfortunately, you won’t be able to see and do everything. And as I’ve already said, after this trip, you’ll just want to come back, especially to explore the east coast.