Potato Harvest in Washington last from Sept-Nov. they are always looking for truck drivers to take spuds from field to storage( it is ag so no CDL required).

Hay harvest is a big deal also, it lasts from May-Nov. Places like https://www.anderson-hay.com have big harvest crews.

https://www.capitalpress.com is a NW ag newspaper that has job postings.

Also a cool seasonal job is firefighter for Washington Department of Resources or the US Forest Service. A lot of my friends when I was in college loved fighting forest fires in the summer.