I am not sure if Denali NP interests you but the colors should be phenomenal at that time of year. And the moose rut might be something you could see there.

Do you already have a car reserved? Do you want to walk on a glacier?

When on Sep 6 do you arrive? Maybe something like this:

Sep 6 arrive Anchorage, overnight
Sep 7 drive to Denali, stopping inTalkeetna for lunch (stop in Anchorage or Wasilla for supplies - you need to take all your own food, drink and water into Denali NP on the shuttle bus the next day)
Sep 8 take shuttle bus (NOT tour) to Eielson or Wonder Lake. My preference is to book a shuttle to Wonder Lake. You can always get off at Eielson if it is cloudy, you are tired/bored/etc. This shuttle MUST be booked in advance.
Sep 9 drive toward Palmer and the Matsu Valley. Lots to do/see including Wasilla (Iditarod HQ), Independence Mine, Muskox Farm in Palmer.
Sep 10 glacier hike with Mica or Nova guides, drive to Anchorage, return car
Sep 11 take early enough transportation (train I think?, or a shuttle service) to Whittier, take Prince William Sound glacier tour before boarding your cruise. This will get you a lot closer to glaciers than anything on your cruise

Another option is to head to Seward instead or as well. If you did it as well, it would mean either an expensive one-way car rental with Avis or a pricey taxi from Seward to Whittier ($200+). On Sep 9 drive back to Anchorage and return the car, Sep 10 early train to Seward, 11.30am Kenai Fjords NP boat tour (6 hours). Sep 11 you can see Exit Glacier or Sealife Center before taking the transportation to Whittier (contact PJ's Taxi). If you leave Seward early enough, you will also be able to take the PWS tour in Whittier.

If you drove, then Sep 9 for sure drive to Girdwood (a shorter drive on Sep 10). Sep 10 continue to Seward, 11.30am Kenai Fjords tour, Sep 11 you can see Sealife Center or Exit Glacier on your own in the morning, leave Seward early enough to get to Whittier for the PWS tour. Note that the ONLY option to rent a car in Anchorage and drop in Whittier is Avis.

A quick check showed:
Avis pick up Anchorage Sep 6, drop off Whittier Sep 11 costs $735 (!)

Alamo pick up Anchorage Sep 6, drop off Anchorage Sep 9 costs $136 then a two-day car rental with Avis pick up Anchorage Sep 9, return to Whittier Sep 11 costs $350, for a total of $486.

I think $350 for the 2-day car rental is a great use of your time even though it sounds expensive. If you took the train to Seward, that is $75/person one way then you still need to get from Seward to Whittier. With the car, you can get BOTH the Kenai Fjords tour AND the Prince William Sound tour. Kenai Fjords emphasizes marine wildlife though they stop at a glacier for about 1/2 hour. Prince William Sound emphasizes glaciers. Both have 2-for-1 coupons in Toursaver and/or Northern Lights books, but you want to check with the operators before you buy a book to see if they still have availability for using the coupons. Most operators limit the number of coupons they accept in any season or on any day.

The only thing I do not recommend is driving from Anchorage to Seward in one day. Too many variables could make this trip a very long one - road construction, accidents, mudslides, slow RV's, etc, etc.