I just got back from doing almost the same trip as you're planning. I, too took Amtrak to Portland (from St. Paul, MN) and found it a neat way to travel. I started in Astoria, Or and finished in Lewistown, MT. From Missoula I went onto the section 5 map, but here's what I can tell you up to Missoula:
-lots of waterfalls on the Oregon side. Multnomah Falls is the most popular but some of the others (such as Latourel, Wahkeena and Horsetail) are very nice also. The Historic Scenic Columbia River Highway is very scenic with lots of shade trees for comfortable riding. Several neat overlooks of the Columbia Gorge.
-Rowena Crest has some wonderful views of the graceful switchbacks falling downwards into The Dalles.
-The Dalles has some neat murals on some of the downtown walls.
-I like the scenery from Walla Walla to Waitsburg.
-Also some good looking country along Alpowa Creek just before Clarkston.
-Long uphill climb from Culdesac to Winchester, but also quite scenic. Nice campground at Winchester State Park.
-Beautiful, beautiful ride along the Lochsa River. Sometimes hard to fully enjoy as Highway 12 has a fair amount of traffic (including trucks) and has no shoulder...ride aware.
-East of Biggs services are rather sparse (with the exception of Umatilla). This stretch contains an absurd number of discarded soda bottles filled with pee, more so than any other stretch of road I've been on. Crow Butte Park has plenty of room for camping. They also have a small store with snack type stuff located in a building at the swimming beach. They just opened as I was there, hours of operation may vary.
Hope you have a wonderful ride,