I believe the street vendors near Jongno 3-ga only operate during the evening hours. So I think if you were to walk there during lunch time hours, it would just be a regular street. But yes you are correct, you can easily walk there from Insadong.
As for Hongdae, wow lots to eat here. I usually come out of exit #9, then make an immediate left and walk down the street until it ends. Then make a right and start heading south along Eoulmadang-ro. You will see tons of different eateries on both sides of the street. It is very pedestrian friendly. I will list a few restaurants and links to things to eat in Hongdae.
Indinine. Indian food. Quite good!
Bonjour Hawaii. Yes, hawaiian food. Located just next door to Indinine. There is a nicer seating area upstairs.
Castle Praha. Czech food and they brew their own beer.
Cafe Wonderland. But I'm not sure if they are still open.
Oi. Another I'm not sure if they are still open. But look at the photos.
Hello Kitty Cafe. A bit overrated I think, but still nice to see.
Korean restaurants and other places to visit in Hongdae.
Yookssam Naengmyeon A small dish of bbq beef along with a bowl of cold noodles. Very refreshing during the hotter months in Korea.
As for bars, although the name sounds funny, "Ho Bar" is fun to see once. It is usually loud, dark, and packed full of people. Despite what the name implies, it is a western-type bar. The only activity I've seen there were people drinking and having a good time.
Hope this helps. Hongdae is a very safe area, I've never had any issues with safety. Feel free to explore the place, there are lots of hidden cafes, stores and such just a few streets in. Oh and don't forget the camera! Trick Eye museum will be loads of fun, but Hongdae in general is also interesting to see.