I live in Gyeongsangbuk-do too-
I'm not exactly sure about must see but ...
Mungyeong Saejae near Mungyeong is nice - You can "hike" (walk up an inclined path) through the forest to a few different gates. There's also a replica of a traditional village which is used in some historic dramas (2,000 won entry fee for the village).
This weekend there's a ceramics festival there and in the fall there's an omija festival (September?) and Apple festival (October)
Hahoe Village outside of Andong is a real traditional village - people actually live in the village and maintain their thatched roof homes. It can be a bit confusing which homes are actually open to visitors and which ones are private.
Andong has a mask festival in early October - groups from all over the world come to perform, you can make your own mask, try Andong soju, etc.
I'd be curious to see what other things people recommend, I'd like to get out and about in Gyeongsangbuk-do more.