In case you doubted my word about Spam for Chuseok before, here's an oldy but goldy blog post about it: yeah.
And in case you didn't have any Chuseok plans yet, and because you're hooped for getting out of town now, in case you're stuck in seoul, because tickets have been sold out for about seven weeks already, as another expat whose gotten stuck in Seoul before on Chuseok, when you can't be sure ANYTHING will be open, let me give you some tips about the best things to do on Chuseok:
1. Go to Namsangeol Folk Village. This is the Folk Village right near Korea House, right near Chungmuro Station, right near the bottom of Namsan (Nam Mountain) right near downtown Seoul. Every chuseok they have tons of stuff to see - performances on the stage, activities like making songpyeon or your own paper-mache hanbok doll, and the like. There's lots to do, and a lot of demonstrations of traditional Korean arts. The park isn't too big, and the stage area has a lot of seating, but it might help to reserve a seat: a few times I've gone and had standing room only.
2. Climb mountains. Particularly the busy ones.
One of the genius things about Seoul, that's never promoted in the Hi Seoul promotional materials (stupidly) is that there are about twenty great mountain hikes, ranging from "I could do this with my step-mother" to "better bring your climbing gear" in difficulty, all within reach by the Seoul Subway and Bus System. Public transportation still runs on Chuseok, as do the odd taxi, so you can definitely get there, and they're mountain trails: it's hard to close those, isn't it?
Head up to the north end of the #4 Subway line, choose a peak, and strike out for it, get up to Uijeongbu and do likewise; check out this list of mountains in Seoul, or this one, or this one. Or do Bukhansan, which holds the guinness world record for busiest mountain, meaning it's the one mountain in the world where climbing it will stress you out, or the one mountain you SHOULD climb if you like being around crowds. Yes. It's the COEX of Mountains. But on Chuseok, there will be fewer people up there than any other day, because most folks are with their families. So take the chance... and September to October are PERFECT climbing weather in Korea. And Koreans are seriously NEVER more pleasant than when they're on the mountain - it's one of the sweetest aspects to the culture you'll ever find.