Hello and welcome to the RTA forums !

With the time you have available you should have plenty of time to explore the 2 parks and there is so much info available when you get there. To find hidden gems it's always worth asking fellow campers and Rangers about there favorite spots in quieter locations. For me the highlights in Olympic NP were Hurricane ridge, Hoh rain forest, Ruby beach and Sol Duc hot springs. The hot springs were more comercialised than I'd hoped but it was a pleasant walk through the woods to the waterfalls. Lake Quinalt has a rain forest trail and Merriman falls nearby as well as claiming the worlds largest Sitka spruce.

In Mt Rainier the views form Rickseeker point are a must and the many stops along Paradise valley. Stevens canyon has lakes and waterfalls and both have many hiking trails.