Dear UBB,
While I don't have any current information on this year's run sizes, etc... I can give you a general overview of what I remember from the 20+ years that I lived in Alaska. First off, you have picked a great time to go - especially for silver salmon. Most of the streams in the Anchorage area should have lots of chums & silvers present by September, unfortunately this doesn't include artic grayling as they aren't found in good numbers south of the Alaska range. If you want to catch a grayling, you will need to drive north on the Parks highway to near Cantwell (3 hrs north of Anchorage). Most of the lakes and streams in that area hold grayling (Pass creek, Jack river, Upper Chilitna, etc...) but you will be driving by some excellent salmon streams to get there.
If I were you, I would target silver salmon in the streams which flow into the Susitna River drainage north of Anchorage. While there may be crowds, you can usualy avoid them by getting up early or hiking a little. I have had the best fishing of my life in this area with 50+ salmon days on many ocassions in September.
Overall, the Montana creek region (97 miles N. of Anchorage on the Parks Hwy.) should be your best bet for lots of fish and fewer people than the Kenai. Here you can fish Montana creek, Fish creek, Sunshine creek, Cottonwood cr. and at least 5 other streams all within 25 miles of the Montana creek bridge and take in some awesome scenery. Each of these rivers should have tons of bright silvers present along with some chums, pinks, resident rainbows and dolly varden.
During the day the salmon bite is usualy slow but I have had success using green, white or orange streamer flies, fire orange mepps spinners (#3), pixies (red insert) or silver vibraxes (#2's) and dry flies or single salmon eggs for rainbows and dolly's. However,the best method to catch fish (and avoid the crowds) is to get up early (3 am) and rig up fresh salmon eggs on a single #4 red gamigatzu hook (with an egg loop) and work the lower stretches of river near where they enter the Susitna. Talk to the locals and move around and you should do well. Good luck!!