I've been "Van Camping" on the south side of Mt. St. Helens for the past 5 years, N of Cougar WA. Great hikking areas and good photos. I also like Clear Lake (SE of Salem on the other side of the pass), and Tillicum on Mt Adams. And you?
If you like quiet try Park Meadow, not a campground. It's off 22 on the west side of the pass. Another quiet spot is Tule Lake, again off 22 on the west side and not a campground. Above Sweet Home there's a nice place with a couple site (not a campground) on quartzville creek. If you're wedded to campgrounds there's Yellow Bottom in quartzville creek area. Little Crater Lake in the Mt Hood area, with a very nice meadow, and good hiking.
For a different experience head for Pine Mountain. It's out of Bend towards Burns. You turn south at the Milicome Store. Climp to the top where there's a no fee campground, no water either, no garbarge service, but there's potty. Across from the campground is the Pine Mountain Observatory. It's open to the public on the week-ends, go look through the 24" scope at the stars, or planets.
As I said someplace else, my favorite camping spot is the one I haven't camped at yet or the last one I camped at. We like to explore so we very seldom go to the same place twice.