I would get a Benchmark Atlas, they are the best road maps for the western states. We've gone through 3 in 6 years out here, just driving roads to see where they go. Highway 20 is amazing, as are any of the roads that cross the Cascades. You really can't go wrong , just don't accidently get lost in a tree farm. The forest service pages are super helpful when they are updated about road closures and conditions. But I always fall back on the Benchmark for all our adventures. And if you are wanting to hike
www.wta.org is the best resource for trails in WA hands down. Detailed hike descriptions, updated trail reports (and roads to get there). You can search by region, distance, anything really.
Bang for your buck you should check out the road to Twin Lakes (NF 3065) off the Mount Baker Highway. Its a fun road to get up to the lakes, and an amazing fairly easy hike to a lookout once you get there. I would try going on a weekday if possible, or at least early as ppl park like idiots on the road if they can't get their rig all the way to the top. You can totally make it in yours, you'll just have to make a few extra turns on the switchbacks. There are camp spots all around the lake and you can camp in the lookout (its first come first served). Pit toilets up there too. If its not fully accessible it should be soon. We always take out of towners up there, even the ones scared of heights. Another fun road is the road to Harts Pass (NF 5400). We love heading out there, usually less people, and you can hike right out of the camp grounds along the PCT. It gets you up high in the mountains so the hiking is easy at that point.