I would recommend working in a trip on the Richardson Highway between Fairbanks and Valdez. It takes about 8 hours to drive the whole thing, and the mountian scenery from Delta Junction to Valdez is absolutely spectacular. I cannot emphasize it enough. Then you can take the ferry from Valdez to Whittier and Seward is only an hour or so drive from Whittier.

We went last May and did the following itinerary in a week:

Day 1 Land in ANC
Day 2 Drive to Denali, explore Denali
Day 3 Explore Denali, drive to Fairbanks
Day 4 Drive to Valdez, explore area
Day 5 Ferry to Whittier, drive to Seward
Day 6 Kenai Fjords Tours glacier and wildlife tour, explore Exit Glacier
Day 7 return to ANC, explore area
Day 8 fly home

You can definitely add some padding in between many of the days to flesh out your extra time, there is plenty to see. But this is, IMHO, a great selection fo what Alaska has to offer.