Dongdaemun's SPALEX is the best! It's cheap, clean, spacious, and comfortable. The location is also incredibly easy to find. Dongdaemun Design Plaza is right across the street. You can find it immediately by taking the elevator down to the 3rd basement floor of the building. It's a good idea to bring your own shampoo, conditioner, body wash, and scrubbing towel, as they are not provided. If you forget, it's convenient that they are sold at the entrance. For women, mugwort steaming is highly recommended! It costs 2000 yen. It feels incredibly good and makes you sweat a lot. After that, a body scrub is the best! On the 4th basement floor, there's also a food court, a jjimjilbang (Korean sauna), and a resting area where you can relax. Going in the morning is recommended as it's less crowded.