Best day trip from Seoul for solo traveller looking to escape the craziness of the city?

I’ll be in Seoul for 8 days and 7 nights during the first week of October and I’m looking to plan a day trip to either a beach, hike trail, or rural town to have a lowkey day? I haven’t seen too many recommendations other than Suwon for a day trip so I thought I’d ask here! Any good recommendations??