I can't see where anybody has suggested this, but I'd reverse the order of your trip and head directly to Sandpoint first via an inland route. If you head south along the WA and OR coast on your return, you're on the ocean side of the road and it's easier to pull into scenic overlooks.
For the quickest route out, I'd suggest I-5 north to US 97 with a stop at Crater Lake and then continue up through Bend (lots to see and do around there) to I-84 to I-82 to US 395 to I-90 to US 95 to Sandpoint.
Returning from Olymipic NP, I'd try to get a visit to Mt. Rainier and Mt. St. Helens before heading to the OR coast and south. One can spend a lot of time along the OR coast and there are amazing SP CG's along the way (although you may be a little late in getting reservations at many of them for this summer). Depending on your timing, you can get as far south as the redwoods and pick your route south from there, either more time along the coast or heading inland to I-5 for a quicker route home.
I don't know how much time you are alloting for this, but I'd say three weeks minimum if you want a fly-by tour of your proposed route. If we had a better idea of how much time you have and what time of year you are going, we could give a lot better recommendations.
