I would absolutely NOT suggest that you cram in both the Kenai and Denali. It could be done of course, but why would you? You won't enjoy either place if you're running around like a chicken without a head.
The trick to AK is to take it easy. Officially, driving time from ANC to Seward is about 2.5 hours. It took us 6, and we didn't stop by half the places we wanted to. And I'm only talking about road pullouts here - you'd need a full day to do the drive properly including Alyeska, Girdwood, etc. My point being that driving in Alaska is not simply getting from point a to point b - every single road is a destination by itself.
I would say stick to the Kenai, and maybe head to Matanuska Glacier on the last day for a guided trek or ice climbing (this was the highlight of our trip).
Spend a nice relaxing day driving the Seward Highway, take a cruise out of Seward to Kenai Fjords National Park (tremendous amount of wildlife, and the amazing Aialik tidewater glacier), or a cruise out of Whittier (more glaciers, less wildlife). Kayak Resurrection Bay out of Lowell Point, hike around Seward, check out Portage Lake and the salmon run, take the Alyeska tram or do the Girdwood hand tram hike.
The last day take the STUNNING Glenn Highway east out of Anchorage to the Matanuska Glacier, it's something you'll never forget.
OR you could take a cruise from Seward and then spend two more days driving north to see a big mountain - because if you do try to cram in Denali, that's just about all you could do in that amount of time. (I guess maybe you could take a Denali sightseeing flight out of Talkeetna, but personally I'd rather do Matanuska and save Denali for next time).
TL;DR - focus on one region and you'll have a much better time.
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