If you like science stuff, I’d recommend Goldendale Observatory. Check their website for times. You just need to pay for parking or have a Discover Pass. I love it there, and Troy Carpenter and Peter Sinclair are awesome at their jobs.

Astoria, Leavenworth, hit cape disappointment when you're in Astoria. If you wanna hit Eastern Washington, look for falls, there's several depending on how far east or whatever you want to go. Have heard great things about the 'ape caves' but haven't been myself. The drive from Yakima along highway 12 is great, but can be a litte dangerous depending on how snow goes next month ... but based on Oct. that is likely not a problem. There's skiiing at the top, but not necessary.

IDK or care where the fuck you go, but make sure your car has tires for snow and ice. Also expect most mountain passes to be closed. Carry snow chains and know how to install them on the side of a highway with logging trucks blasting by you while you’re laying on your belly in slush on the shoulder of the highway. Oh and pro-tip? Practice putting them on in a garage or carport before you have to get stuck up in the mountains trying to figure shit out.

I recommend the Olympic peninsula and coast in winter - historic Port Townsend, Dungeness spit near Sequim, Lake Crescent, quirky Port Angeles, Cape Flattery (northwesternest point of the lower states of the USA), the beaches along the coast - Ruby, Kalaloch and the numbered beaches in between, Hoh National forest for mossy trees, the inn at Quinault… all beautiful in winter light and no snowshoes needed.

Make sure your car has winter tires though, it can get very slippery during winter.

Rialto Beach is amazing when a winter storm rolls through. Just be careful, the waves can toss up logs.

This is so vague only vague answers can be given…

My recommendation is you learn how to make an itinerary

It really depends on the type of experience you are looking for. By December, there will be snow in the mountains, while the coast and lowlands on the west side of the cascades will be mostly dry. If you are looking for outdoor recreation, what activities do you like. Or are you more interested in food/culture tourism? Where are you going to start, how much driving do you want to do, and are you comfortable with winter driving?

If you want something interesting, go down to marry hill and see Stonehenge and you can get around a lot of some of the passes and gain access to the eastern side of the state