Olympic National Park is beautiful, and the rain forest near Forks (of Twilight fame) is amazing. If you do go all around the coast, Kalaloch is a nice campground - more rustic than some, but on a great bluff over some amazing beaches.
https://www.recreation.gov/camping/k...O&parkId=70944
But... if you're limited on time, I'd aim for Mt. St. Helens first, and Mr. Rainier second.
One downside of Olympic Nat'l Park, if you travel with dogs and like to hike... no dogs allowed on trails in the Park (if you're not a dog person, I guess this would be a plus?). They are welcome at the campgrounds and on the beach, but not up the rainforest trails due to wildlife concerns.
PS - the pic in our signature, below, was taken at Kalaloch.