STP Gathering 2010
I like Widerstand's suggestion of Manzanita. I've been there a few times and it would be really easy to camp out in the dunes without anyone knowing you are there. I've been out on the beach at night plenty a time there, and I have never seen a cop. There are always people drinking there, but it is such a spread out beach that it isn't worth their time to investigate. Plus they would never see you or a fire in the dunes, especially if you were far enough from town.
Personally I think hot springs are a bad idea. At the better known spots camping is always illegal and so is drinking. In OR, the most popular springs are always on patrol routes. Of course we could go with a lesser known spring, but there are problems there as well. First of all, I don't know how easy it is to find a lesser known spring that is semi-easily accessible, and second of all its fucking dangerous lol! NEVER get in a hot spring that you aren't absolutely certain is safe (and you won't know this if it isn't popular). Shit, not too long ago some people died in one of the popular redneck springs out here. The temps. can rise suddenly without warning and fucking cook you. So good idea in theory, but in my opinion way too sketch to actually do.
Throwing out a couple more ideas, river camping is always fun if the beach doesn't work out. Plenty of rivers to choose from to. There are plenty of good spots on the Clackamas river that I know of, and I've heard a lot of good things about other rivers in OR. Ideally you want a spot off of a backwoods highway where the river bank is below the road. There's a lot of switchbacks from the hwy on the Clackamas that take 5 minutes to reach the bottom. Some of them have a lot of space at the bottom, and there is no way to see it from the road, and no way a cop is going to go down and spend 15 minutes checking each of them (there are a lot over a long distance). Personally I've grouped camped out there with a bunch of stoners getting fucked up and blowing up mortars. If that doesn't get you caught, not sure what will and we were fine. Something to look into. Pretty much spots like this all over OR and CA.
Some of the northern beaches in OR might be worth checking out (Astoria and such), but those tend to be a lot more popular than some of the southern and mid beaches. Really, Arrow is right about getting permission to use someone's property. That's a pretty safe bet for something like this. I'm running locations through my head, but haven't thought of anything so far. Only decent place I have access to is a joint-venture cabin in AZ, but distance aside it would likely be snowed in til summer.