The Oseberg Royal Mound is a quiet field landmark with an outsized place in Viking history. The famous Oseberg ship, now one of Norway's great archaeological treasures, was excavated from this burial mound near Tonsberg in 1904. The mound has been reconstructed, so the visit is more about standing at the source than seeing the artefacts themselves.
Go with the story in mind: two high-status women, a richly equipped ship burial, and woodcarving so fine it reshaped modern ideas of Viking art. The mound pairs well with Slottsfjellet and the Viking-focused sights in Tonsberg. It is modest on the ground, but important in context.