What's the name of the apartment? I know of a place called Inn The City by Nonhyeon Station that's really popular. Actually, my friends and I drink and hang out at the CU Mart underneath it all the time haha. Very nice area, and it's just across the river. About ten minutes closer to where you want to be than past Gangnam.
The Gangnam area is only busy at night behind the Gangnamdae-ro. Like in between Sinnonhyeon and Gangnam Stations there are alleys that run parallel to the main street. Those will be full of activity. You might find, outside of there, that at night on weekends, it's quiet.
Also, behind Sinnonhyeon Station is an area called Nonhyeon-dong which is wildly popular at night. This is close to Nonhyeon Station I mentioned, of course.
To be honest, if you're only here two nights, don't go all the way to Gangnam. Time will be limited. Stay in Myeongdong, catch the AREX (you won't have light bags). If you go to Gangnam, you'll end up taking an airport bus , which means traffic and extra time wasted.
In Myeongdong, you're smack in the middle of Seoul, so you can get anywhere easily. If you stay in Gangnam, you're going a lot of extra distance to everywhere you want to go.
All the tourist sites and shopping you'll need will be right there in Myeongdong and Jongno. Gangnam shopping is the same as Myeongdong. Chain stores and a department store. Don't worry.
I don't know what kind of 30 you are haha, but if you're feeling young, late night adventurous, I'd go to Hongdae. It's THE youthful/trendy area for clubs and indie stuff. Itaewon has now become a trendy global area for Koreans to go to fancy foreign restaurants run by expat Koreans haha. Think American barbecue, fancy Southeast Asian food, juice bars and churros. If you want something slightly more upscale cafe, check out Garosu-gil.
Myeongdong has nothing at night, though. I'd head to Hongdae, Nonhyeon, Itaewon or the alley behind Gangnamdae-ro. Both will be about a 7-10,000W taxi ride each way (traffic depending, of course).