Cougar Rock is a car camping site, but I guess they have a walk in site. A few sites have very large old growth trees. September should be ok, can be busy and noisy with the road right there. The walk in site I think is the furthest in. I camped there allot.
Most of the BC sites are more concerned with limiting impact so down lower in forest etc, which you can tell from river camps. How are you getting around the park?
I think you are doing the wrong half of the wonderland, it should be nice though if the smoke goes away.
Just at that one campground, unless you are lucky, but I camp there often, esp in the spring
don't stick your thumb out when a Park Ranger is passing by it isn't allowed (I know, he let me off with a warning). I suppose you could get a ride clockwise around the park. Get a ride with someone to 410, then head East then south to the entrance on the SE side, and then a short ride on the road on the southside to box canyon. Not sure how you would go the other way. It might take a few rides to do this, but if you have plenty of time?
Or I guess you might find some hikers who have to go back for there second car or someone who is dropping food off who is doing those roads. check with people at your last camp etc.?
I guess you could dayhikei from Mowich to the Road on the Southside and hitch hike from there? Not sure of mileage though, maybe use the westside road if it is open for hikers. I biked the westside road and hiked the wonderland to above Mowich River and back so it should be doable oneway and you would have an early start since you would already be in the park.
If you like to day hike 20+ miles, I would change your plans to one or both of these great day hikes. Keep your permit until you get there, and then see if you can change it they sometimes have extra spots for walk-ins (or they used to).
Get a permit to Camp 2 Nights at Nickel Creek, get up real early and drive around to the North trailhead, and day hike to your camp. The next morning break camp and hide your gear somewhere, and dashiki to your car. The first time I did this I came back on the Eastside trail, the second time I went back on the Wonderland trail to get more pictures. Both times I had to Camp 1 night at Maple Creek and move my Camp to Nickel Creek, but still doable.
I also did the Northside this way, but had a bike, they were going to open the road the next year or so, but I decided I would rather go when it was less crowded, still plenty of people. I Camped 2 nights at Sunrise, and drove around to Carbon River I think that road is closed again except to foot and bike traffic so you would have to walk it, not sure how far it is. I did the Northern Loop it was really cool as was the Eastside above. Sure I would rather camp at one of the high elevation camps but I did not have to carry a full pack and could do this on a weekend. I got back to my camp gear near dark, so have a good flashlight etc.
I have had to argue with the Ranger about this because you are not allowed to leave an empty camp overnight. But I wasn't;t doing that, I was just driving to a different trailhead, and hiking to my camp - during the day.
The other option would be to camp.hike off trail but if you are not familiar with the park I can not recommend that.
The smoke now is from BC it is really bad but it is suppose to be gone after tomorrow for a while (depending on the wind)
I went to Rainier on Friday, but by that morning they already said the smoke was coming back Sunday or Monday, and it looked ok from Paradise, but hiking the skyline trail you could see smoke in all directions esp to the East. Saturday I worked as a volunteer in the Paradise area, you could see the smoke close in a bit by that afternoon. Today Sunday, I cancelled my backpack, Still allot of people enjoying the park, I just did not think is was worth it.