Waterfall Garden is a really cool and secluded place to smoke depending on how busy it is. It is on 2nd avenue near occidental square downtown.
Discovery Park is another good one. Lots of little secret spots you can duck away at and toke up.
Denny Blaine park on Lake Washington blvd and Bhy Kracke park in queen-anne are probably two of the best kept stoner secrets in the Seattle area. Shhh, dont tell anyone.
Olympic National Park has a ton of great ideas to just hang out and smoke for a chill afternoon. It is such a big area that finding privacy shouldn’t be terribly difficult. Not too far from Forks too. In particular, the Ozette Wilderness trail is worthwhile as it juts out in different areas and can lead to Ozette Lake, which is a nice spot to park and smoke. It also heads out to the Sand Point which will take you to the beach, so plenty of options there. Another good spot would be the Quinault River Bridge trail, which usually doesn’t have a lot of people and is fairly moderate in difficulty. Beautiful spots there to relax and smoke.