http://www.uwsp.edu/geo/projects/geo...Scablands0.HTM for ideas. Dry Falls area is at the top of the list.
If you don't already have one, pick up a Delorme "Washington Atlas & Gazateer" (G.I. Joes, REI etc.) - it includes topographical maps and shows roads, backroads, forest service roads etc.
Edit: Be sure to go to the bottom of the above link and click on the various trips.