Listed as a World Heritage Site in 2014, Tomioka Silk Mill provides a wonderful look into the history of silk production, with excellent English-language narration. Completed in 1872, the mill was once one of the largest producers of silk in the world. Today, its buildings are some of the only Meiji-era government factories preserved in excellent condition. To get here take the Joshin Dentetsu line from Takasaki to Joshu Tomioka Station, then walk 10 minutes.
It's a fascinating day trip from Takasaki.