I'm from Washington - i would make sure to see the San Juan islands if at all possible. Take the ferry from anacortes (beautiful town in itself). Even better if you drive up through Whidbey Island via taking the Mukilteo ferry to the south side. Langley and Coupeville are worth a stop on the way. Deception pass state park is on the north end and is a must if you're in the area.
Friday harbor is the biggest destination in the San Juan's and a fun day trip. You can even walk on the ferry and hit everything in town on foot, although San Juan Island which it is located on is worth checking out for hikes, Roche harbor, etc. The ferry ride through the islands is worth the trip by itself. Would be a day trip, none of the islands really worth staying with a group. Orcas and Lopez are even more lowkey.
In the anacortes/Skagit valley area, Chuckanut drive to Taylor Shellfish farms is incredible (the drive and the food). You can stay in Bellingham which is a bigger city with nicer amenities and from there it is easy to get to North Cascades which I would recommend over Olympic, the Mt Baker wilderness is underrated as well.
In general I would say Rainier is a must but you can do fun day hikes that will give you a great view of the mountain and be out of there in half a day. I recommend getting there as early as possible as it gets crowded especially in afternoons.
Leavenworth (Fredericksburg in the mountains) is beautiful but can be touristy so unless traveling over the mountains on highway 2 I wouldn't make it an absolute priority.
I'm less familiar with the peninsula but be very deliberate if choosing to go there, Olympic NP is massive and you can't see everything in a day or two, and there's really no where to stay once you commit.
With 8 days you can easily see the east side of the state too. Second the Walla walla recommendation for the wineries and the town. Airbnb's are a great option there.
Lastly, Seattle has definitely gone downhill but since you're most likely flying in there and if you've never been, pikes place and the needle are worth the visit. If you're planning on staying there and need something else to do other than the city, the ferries to bainbridge or vashon are good options. Sorry for the novel!