Love Seaside!! We usually go every year. It is worth the extra money because everything in Seaside is within walking/biking distance. You can park your car and leave it there for the entire vacation.
It's definitely low-key - not too much going on there except the beach, eating, and some high-end shopping. We love to go and ride our bikes throughout the town and just take it all in.
Highly recommend it!
Also, if Seaside is too expensive, check out the western side of Seagrove Beach (it neighbors Seaside) and Watercolor (the other).