If you want to go to Frederikshavn in Denmark by boat to Larvik , you can take it for a round trip. It takes about 6 hours. I found a good website when you can choose your ship lines and check at the timetable to decide when you want to live and how much you want to pay.
Look at the site below but remember that it is written in ducth!!
A great way to get to Larvik is flying with Ryanair. This is because the Ryanair Oslo Torp destination lands in Sandefjord, just 25km North-East of Larvik (That means it doesn't actually fly into Oslo - but a destination 2 hours south).
From 'Oslo Torp' you can fly to Glasgow (Prestwick), Liverpool and London (Stansted) in the UK, Bremen and Frankfurt (Hahn) in Germany, Barcelona (Girona) and Madrid in Spain, Marseille (Provence) in France, and Milan (Orio al Serio - in Bergamo) and Pisa in Italy.
All flights are relatively cheap, if you want snacks on board you have to pay for them (but you can bring your own on board if you want). The seats are comfortable enough, but sometimes on flights longer than 2-2.5 hours they can start to feel a bit stiff.
I travelled straight from Oslo airport to Larvik, Vestfold. It took about 2hrs 40mins, I didn't have to switch trains anywhere either (From Oslo S it is 2hrs 7mins). The trains have good storage for luggage - hand luggage and bigger bags, like suitcases. The staff were very friendly and helpful, the seats were comfortable and the views all the way to Larvik (once away from Oslo S) were amazing (lakes and forests).
From Torp international airport it is 25 minutes journey by car to Larvik. Look a the link below to find out some informations about the airport.