The airport is closed to a small but pittoresc town of Sandefjord, a fishing village. It´s pretty nice but you can walk through town in a couple of hours. There´s a whale museum to visit. I think Sandefjord is 1-1.5h from Oslo. If they continue along the cost they'll find many nice small towns and also beatiful beaches, in the bays and fiords. Kristiansand is also nice and I think is further away than Oslo. If they want to visit Sweden, they can take the ferry between Sandefiord and Stromstad, a small summer town in the Swedish West Cost. I think the ferry takes 2.5-3h and is traffiked by Color Line. Good Luck with their trip!