The 6 hour or longer Kenai Fjords boat tours from Seward get you out of the bay. You get much closer to glacier and marine wildlife than you will ever get on your cruise ship. You spend about 20 minutes at a glacier but the highlight of these cruises is the marine wildlife. They are definitely well worth it.

Note that a one-way car rental with Avis will not be cheap, but it lets you use your time much more wisely than paying to get to Anchorage to possibly rent one a bit cheaper. Figure +- $600 for a 3-day one-way car rental. If you made your way to Anchorage and rented from there, you would pay about $250 + the cost to get to Anchorage.