What about SW Oregon, Lost Creek Lake has a large "group" site. It's not far from Interstate 5 and on the way to Crater Lake. There is also dispersed camping near there.
East of the Cascades there is the Oregon Backcountry Discovery Route, or sections of it. It would make for a good "loop" with dispersed camping every night (or campgrounds). You can kind of pick your poison, depending on what section of it or what loop you decide to make. Can start in SE Oregon near Lakeview (Section One, Route Five) and end up near Crater Lake (from Section One of Route Five, over Route 2 near Summer Lake to Hwy 97 or continue from Route 2 to Route 6 over to Interstate 5 for a quick drive (north or south) home).