It's frantic and can be overwhelming, but worth a few visits.
During the day, the shops are a little easier to navigate, as the food vendors don't start up until later in the day. There are lots of similar things but in amongst it, a couple of vendors or shops doing their own thing, so it's worth looking through everything.
If you're looking for street food (which mostly is not representative of actual Korean food, but still fun and tasty) then this is the place.
There are heaps of places to buy Korean skin care and beauty products (Olive Young has many stored in the area, with the flagship store currently being renovated) (Olive Young does tax refunds on the spot for any purchases over 30,000 Won) as well as Nature Republic, Tony Moly, and heaps others, which sell the usual Korean face masks (containing snail secretions through to pearls from 200 to 5,000 Won each - cheaper to buy in premade packs of 10 or more). There are a lots of fashion and sports stores (Zara, Spao, Nike, 8Seconds, etc), cheap jewellery places (The Plain, RedEye, etc) markets (7-11, Korea Mart, etc to buy food, drinks, snacks), places to eat, there's a Daiso, a Mixue, a second hand clothes store (sells by the 100g) pretty much anything you need. There are even couple of ca cafes. And if you walk through to the Lotte store side, you can get to Lotte, Lotte Young Plaza, Avenuel via the substations, which themselves are underground malls.
Lots of fun to be had, although absolutely frantically busy as the street food markets start up. And the tourists are not looking where they are going as they're too busy taking photos.
The only word of warning is that there are very few bins (anywhere in Seoul but noticeable in Myeongdong as you usually have a takeaway plate of food) there are bins at the major entrance and exit points but the easiest thing to do if simply stand to the side near the vendor you bought the food from, eat it and ask the vendor to take the rubbish back - they will all do this with no problems. If you are intending on eating while you walk, take a small plastic bag with you and a pack of wet wipes so you can clean yourself up and contain your rubbish as you go.
I didn't find any public toilets in the Myeong streets are, but there are some in the underground malls joining Myeongdong to the Lotte stores.