Alaska is huge, and the weather is different in different portions of the state. You are going to have to be more specific as to the areas you are interested in.

For most visitors, the main concern in September is access to the interior of Denali Park. Some years the road closes in late September, October in others. The park's shuttle bus service ends this year on September 13, by which time autumn will be in full swing.

If your schedule has you visiting Denali after the 13th, then you could miss out. Of course bad weather can stop a lot of activities, and fall comes early in Alaska.

But a bit more information on your plans will really help.

Sept 7 is not too late. As Gardyloo pointed out you can still access the interior of Denali (I'd make that my first stop since it does start to shut down not too much later and the tundra colors are fading fast by Sept 7) and you can still find excellent bear viewing and access in Katmai at Brooks (for example) and pretty much everything is still open on the Kenai at Seward and Homer.

For most people these are among the main tourist attractions, and you should be fine on those.

Were there other places you wanted to go?

Starting on Sept 7. We will be going on a land tour that starts in anchorage, valdez prince william sound, fairbanks, then to denali national park on the 6th day, sept 12, for 2 nights, and back by alaska railroad to anchorage.

Just read the shuttle bus for danali will end on the 13th, this is the day we were told we would be going into the park for 6 to 8 hours.