When in Larvik a trip to the nearby town of Stavern is a good idea. It is a beautiful little town with a great waterfront. For more information and a couple of photos, check out my Stavern Travelogue at the bottom of this page.
Kaupang is an excavated Viking town. The people at the village (in traditional costumes) were more than happy to tell us about how the vikings lived, most of them likely to have been craftsmen. Nearby there was also a grave (but being a rainy day I didn't feel like venturing through the mud).
Well.. maybe not a 'must see' activity, but if you're interested in boats, then Colin Archer is your man (or was your man). The tourist office will have more information.
This quote, however, also seems to be appropriate: "In the town's little skerry park, Tollerodden, a stone's throw from the nautical museum, are placed statues of both Thor Heyerdahl and Colin Archer, reminding us of the great significance of the sea has had for Larvik." -http://www.norwaypost.no/content.asp?cluster_id=9226&folder_id=18
Larvik is a fishermen's town so at the harbour you can see many boats that goes up and down along the little fjord. I like staying a bit there looking at the sea and I discovered many wonderful "glassmanette" aka jelly-fish in english ;-). I apologize for my poor norwegian!!
Not far from Larvik there is an archaeological site when students of archaeology can work and discover old viking stones and things. The site is very close to the center of Larvik so it's easy to find it, you can ask to the local people.
A roadsign on the 40 meaning that there was something to be seen of the road made us make a right turn. After a short walk we ended up at this waterfall "Kjaerefossen".
It was a great sight, lots of noise too. It is a spot for catching salmons. There was also a fish stair