The olympics can be challenging because of how it's laid out and the road network- you'll do a lot of "out and back" driving unless you can wrap the whole thing (something I highly recommend) but that can be hard logistics.
On the interior side of the peninsula, I like to ferry over to Bainbridge, then to poulsbo and across the floating bridge. From there, you can either go down the hood canal arm for a drive along the coast and see some small cool places like the Dosewallips and hama hama oyster company, or head up towards Pt. Townsend which is absolutely worth a day (gotta hit Ft. Worden while there). Dungeness spit in Sequim is cool with low tide, but then on to Hurricane ridge. After that, the Hoh rain forest is pretty awesome and you can wrap all the way around to the coast and the furthest NW point in WA.
Plenty of folks will talk up the N. Cascades, and it is great- Stehekin, Winthrop, etc. But I honestly think the Olympics is one of the most unique landscapes in N. America. Geologically young mountains, a giant fjord, a temperate rain forest, and maritime small towns....pretty cool place with lots to see.
I'd also say the Olympics are much less crowded in my experiences- there's just more area and less people funneling to the same places at the same