The Dolenjska Museum occupies the old provost's house in the centre of Novo Mesto and holds the regional collections covering prehistory through to the 20th century. The star exhibit is the Iron Age material from the hillforts and burial mounds surrounding the town: between roughly 800 and 300 BC, Novo Mesto was one of the most important centres of the Hallstatt culture in Europe, and the grave goods recovered from local tumuli — bronze situlae, amber and gold jewellery, decorated helmets, weapons — are of exceptional quality.
The museum also has good coverage of Roman-era Novo Mesto (it was a significant settlement on the Amber Road), medieval and early modern history, and folk culture from the Dolenjska region. The building itself, with its Renaissance and Baroque elements, is worth a look.