Most of these have already been mentioned, but to summarize the list and add a couple:
- Highway 20 out to Winthrop, back on 2 or 2/97/90. If you have more time, loop up through the Okanagan Valley and further out into north-eastern washington (97/20/21/2) Leave early for this trip so you can enjoy the run out 20 without having to deal with too much traffic
- Run Canyon Road (821) out to Yakima from ellensburg, return on 12/410 or 12/123/410 through Rainer National park. The drive from the east park entrance up to paradise is a lot of fun.
- The road up to hurricane ridge - Olympic National park out on the peninsula
- With less time available, hop the ferry over to whidbey and work your way up to Anacortes for dinner on the back roads on the western side of the island. The roads aren't as curvy as some mentioned above, but it can be a nice afternoon of driving
I've been taking about one weekend day a month away from the madness of work, and spending 5-12 hours just driving one of the above routes. 542 up to Baker is next on the list.