You can look at both Phillips and Major Marine for the Prince William Sound cruise. I have found Major Marine boats less crowded and cheaper (due to 2-for-1 coupons) than Phillips.
Depending on when you arrive into Anchorage on Day 1, I might consider two nights in Girdwood. There is so much to see/do in that whole Portage Valley area. Besides what Gardyloo mentioned, there is Potter Marsh (look for moose and birds), gold panning, Wildlife Conservation Center, Winner Creek Trail for the unique hand tram (has been closed for repairs since early summer but is supposed to be operating again soon), Begich Boggs Visitor Center, Byron Glacier trail and more.
Then the last day, depending on departure time, you can spend time in Anchorage. Most flights depart from ANC to the lower 48 in the evening so you would have the better part of a day in Anchorage.