The canal from Skien passing up to upper Telemark. Smaller cruise ships will take you all the way during summer season. Highly recommended!
Ibsen's Venstøp. The house outside the city of Skien where Henrik Ibsen spent a few years of his childhood. The attic is said to be his inspiration to "Vildanden" The wild duck.
Brekkeparken. Nice park with old historic buildings, and a museum.