My first question is, when does your return flight leave Anchorage after the cruise? If it leaves within a day of the end of the cruise, you will not have time to get to Denali. Have you signed up for a tour through the cruise line, or do you plan to try to do this on your own?
If the former, don't worry, they'll get you there & around, primarily by bus unless you opted for rail. If you booked your trip through a travel agent, ask them for assistance with this, and do it pronto!
If the latter, you may or may not be able to make reservations this late in the game, but your best bet would probably be the Alaska RR. It should take the better part of a day, and possibly will require an overnight stay in Anchorage, to get to Denali. You will NEED reservations to stay near the park, and most likely for the train as well. Contact the Alaska RR right away for reservations, and ask them about hotels/lodges near Denali, for starters.
If you are doing this on your own, and can't get the train from Seward, then contact RCCL right away about purchasing a transfer from them to Anchorage. That will likely take you to the Anchorage airport, or possibly to a downtown hotel (ask). From Anchorage you can take the train, take an (overnight) bus tour (Gray Line comes to mind), or rent a car and drive. The drive is probably about 5-6 hours from Anchorage, one way; train will take somewhat longer due to the stops it needs to make. You will not be allowed to drive into Denali NP itself, but the National Park Service and other tour companies offer bus trips into the interior of the park to watch for wildlife (very well worth the time!!) Nearly all Denali area hotels & lodges are situated just outside the park, so you can drive to them.