We rented a car & drove out to Portage Glacier. They have an excursion boat, but we were there too early for it to run (May). It's a nice drive; we also stopped at a ski lodge on the way, as well as an animal park. Some nice restaurants in Anchorage also.
Request their visitor guide- www.anchorage.net plenty to do, but I would skip the museums and head out doors. Eagle River nature center, Elkutna Spirit hours, biking the Coastal Trail . Drive Turnagain Arm to Hope. Definately get a car, you will be way to limited without one.