"Silver falls is a cool trail. Easy and many beautiful waterfalls. But it IS a state parks and so you have to deal with all of the wonderfully inconsiderate zombies bringing their litter of satanic spawn to fester in Pabst driven family outings. The perfect storm of visual and auditory pollution. But I digress..."
While I certainly agree that Silver Falls is among the more crowded destinations in Oregon, I do feel they have the space to accommodate them. You have to keep in mind that crowd size is relative; having come from Southern California, all I could think about when I first moved up here was how uncrowded the trails were! =) Anytime there was snow in the San Bernardino mountains, there would literally be a parking lot on the highway up. I can remember sitting in traffic on a mountain road for two hours, only to get there and wait for another half hour to find a parking spot, just to walk around with a bunch of underdressed city slickers.
There are places in Yosemite that you literally stand for minutes while packs of people pass you on the trail, most of them in cotton shirts.
I got into backpacking because most of my Southern California car camping tips were ruined from people's boom boxes blasting until early morning and obnoxious partying (despite the unenforced quiet hours). I remember telling my wife that I could actually get back into car camping up here because the experience is so much more pristine.
I'll never forget coming across hike 76 in Silluvan's "100 Hikes in Northwest Oregon & Southwest Washington." He states: Avoid this popular hike on summer weekends, when the unmarked parking area is jammed and the trail crowded." But when I first drove past the trailhead, I saw there were sonly spaces for about 15 cars! Sure, it may be hard to park, but I remember thinking "if this is what crowded means in Oregon, I've moved to the right place!!" I'm just saying that one person's crowded adventure is another person's pleasant solitude. =)
Anyway, on to the post. That time of year it is best to stay in low elevation, which means Columbia River Gorge hikes (like Eagle Creek) or Silver Falls. If I were doing your trip at that time, I would personally consider car camping and hiking, and visiting several of the waterfalls. Both Silver Falls State Park and Eagle Creek have world-class waterfalls, and are truly unique and worth visiting. Silver Falls has really cool cabins you can rent for about $30 a night which is really nice if it is going to rain all weekend. But don't let the rain stop you; they are pleasant hikes in any weather!
Silver Falls is a 7 mile loop, but it will take you 3-4 hours to do it with all the stopping and gawking you'll be doing. However, it is stunning; several of them are more than 200 feet high, and you get to walk behind 4 of the waterfalls. The trail is in a narrow canyon filled with vine maple and Douglas fir; in the fall the maples turn crimson red. There is also a variety of activities; they have a 4 mile paved bike loop through the forest, a swimming hole, large, grassy meadows that should be filled with wildflowers that time of year, and a 14-mile perimeter ridge hike.
Eagle Creek, on the other hand, is blasted out of a cliff edge much of the time, and is a bit longer at 12 miles. The views are dramatic at times, and it will take you 6-8 hours to get to and from Tunnel Falls (the highlight of the trip). If you wanted to backpack, this would be the trip to do so; the trains at the trailhead campground will keep you up at night. But I would plan on an in-and-out, as the ridgelines may still be under snow and it's about a 35 mile loop.