I'm new to Seoul! What K-beauty treatments and beauty experiences would you recommend?

Hi, this is Aya! I'm so excited because I'm going to Seoul next month! I've always been interested in K-beauty, and I'd love to actually go and experience it. I want to check out skincare stores, beauty clinics, and all sorts of other things.

I'm especially interested in visiting Korean cosmetics flagship stores and trying out sheet masks and base makeup products, but I'm wondering if there's any difference between trying them in person and buying them from Japan? Also, I've heard that there are areas like Gangnam and Myeongdong where beauty facilities are concentrated, but which would be best for a first-timer?

Furthermore, I'd love to know if there are any places where I can easily try out skincare treatments. I'd prefer something affordable, not expensive. If you've actually been there, I'd love to hear about your experiences and what you liked!