I've been to Sakata, but by car, so different logistics to you, plus
the day I was there happened to coincide with a marathon race that
closed roads in the downtown area and prevented me getting to several
spots so I cannot give a comprehensive overview.
The most useful online general guide to Japan is https://japan-guide.com
Here's their piece on Sakata https://www.japan-guide.com/e/e7990.html
and unfortunately it is not very good. It misses Somaro (Maiko
performances) and Kaiko-ji (mummified priests) and Hiyoriyama Park
(lighthouse and reconstructed ship).
This trip report covers most of the good sights: https://www.japan-guide.com/community/mfedley/report-3272
but misses the park: https://yamagatakanko.com/en/attractions/detail_402.html
Other good sources are the JNTO pamphlets https://www.japan.travel/brochures/eng/index.php
Here's the one for the Sakata area, see items 5 and 6 on p3 and 4 on p4 https://partners-pamph.jnto.go.jp/simg/pamph/2129.pdf
And the Japanese ports directory: here's Sakata https://www.mlit.go.jp/kankocho/cruise/detail/015/index.html
and note the pdf link in there for more info.