Huge outdoor park on an archaeological site where ruins from the Yayoi Period were discovered and excavated since the mid-1980s. The Yayoi Period (10th–3rd century BC 弥生時代) is noted for the start of rice cultivation in Japan. Yoshinogari is Japan’s largest known Yayoi archaeological site and a Special National Historic Site (特別史跡).
The park recreates a Yayoi-Period settlement with homes, worship halls, storehouses, a watch tower (right photo), and even a graveyard with large clay pots as coffins (left photo shows recreated graves).
You can go inside most of the structures where you might see mannequins mimicking ancient rituals. There are also modern tourist facilities including a restaurant, gift shop, and recreational area.
No problem social distancing in this huge park. Admission is cheap (¥460 for adults). Open 9 am to 5 pm or 6 pm in summer. Closed on Dec. 31 and the third Monday and following day in January. 
Short walk from JR Yoshinogari Koen Station (JR Nagasaki Line), not far from JR Saga Station.
Official: https://www.yoshinogari.jp/en/