What kind of summary are you looking for? Olympic is very diverse and spread out. There's the beaches on the coast, the Hoh Rain Forest, and Hurricane Ridge. Lots of driving, and needs 2 full days minimum with driving to see it all, but very unique and fun. We went to 2 beaches (Shi Shi and maybe 2nd?), the Hoh Rain Forest, stopped at a beautiful lake, and loved the Hurricane Ridge hike.
Mt. Rainier is beautiful, but not as much "to do" other than various hikes and vantage points of the (beautiful) mountain. North Cascades is amazing and doesn't get the credit it deserves, especially for being so close to Seattle. We spent the better of a day driving the main roads through the park, getting out at viewpoints, and doing a lovely 1-2 mile hike to a lake surrounded by mountains. Hope that helps.