Especially in winter, The jimjilbang (or korean spa) is a good way to relax and enjoy some warm public bathes and saunas. Usually they are however not very foreigner friendly since all the signs are only in Korean but it's not the case of dragon hill. Located just next to Yongsan Station (exit 1, spot the golden dragon on your right), you will be able to take a swim in the outdoor swimming pool, enjoy diverse saunas and take a rest in the public bath (full nudity expected there) for 11000 won (around 10$) a person.
If you want, you can get a massage as well (they will offer you this kind of pack at the entrance but they are pricy so most koreans don't do it) or simply get a 10 minute chair massage for 2$ in the common area, also nice. And if you come with kids, they can enjoy all sorts of games as well as the outside swimming pool (bring your swimming suit!) while you're resting in the sauna.
There is also a korean restaurant (dishes around 7-10$) and all sorts of snack and drinks available. If you have questions, the staff will be able to help you in English. So don't expect the luxury and quality of a western spa (for the price, it's quite normal) but more the korean public bath. A good way to spend a few relaxing hours in a typically korean style.
Last piece of advice, they had some thiefs breaking in before so if you can, leave your valuables in your guesthouse/hotel. (I however never had any kind of problem there before).
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