Personally, I’m more in favour of the sea. Lying on the beach with a good book as companion would suit me just fine. Unfortunately, mountains are much closer than the sea, so for a weekend trip, mountains it is.
That had been the great thing when I lived in the States; just an hour’s drive to the pacific. (We used to drive half an hour in our lunch break to grab a cup of coffee from a shabby booth in the next town, so an hour to the sea could still be considered very close). Sure, you’d freeze to death if you’d tried to go swimming, but nevertheless it was great to listen to the sound of the waves, seagulls, and to let the dog run, drink saltwater and vomit into the car on the way back (first time, only).