Ellen -
I just read an article about the clean-up progress around Rainier, and while things are improving, there is still major damage from last year's rains. August is still a ways off, so I'd keep an eye on the NP website for updates as the time gets closer. I like the other visitor center as well, over near the White River Campground, but IIRC, the road won't be open there this year due to the damage.
To answer your question about AMC-like groups - yes, their counterpart in Washington state is the famed Mountaineers, and they have local chapters, so maybe you can scout up something via the web. Oregon's counterpart is the Mazamas, which I mention here because that state isn't too far away. I had the pleasure of hooking up with the Olympia section of the Mountaineers the first time I did Adams, and climbed with them the last mile to the summit - a cloud bank had rolled in, but the leader was very familiar with the route, and he allowed me to join them.
What to do while Brian is on the mountain? Well, you could climb
Mt St Helens . That mountain re-opened last summer, and I did it again in early September. A climbing permit is required, and unlike the 'old days', you can't show up at Jack's Store the night before and get an unclaimed reservation. The best option is to drive to the trailhead (Climber's Bivouac) the afternoon before, camp (it's free) and get an early start (about 7AM is recommended). You be down around mid-afternoon, and you can either camp another night or head back to Paradise. More info on Helens is
here.
Mt Adams is also a possibility, but it's another 2 or 3 hour drive from Rainier, and is usually done as a 2 day climb. It's in a remote area, so if you're doing it solo make sure your navigational skills are in good order.
And, if you haven't seen the many fine sections of the Columbia River Gorge, especially from the Oregon side, you can have a most enjoyable day at the many state parks along the way as you head East towards Hood River. And, don't forget to watch the wind surfers in Hood River - always a hoot - and drive back along the Washington side of the Columbia.
PM me if you'd like details on any of this.
Kevin