I would avoid Holland America if you can afford it as I did not think they trained their staff well or for that matter gave a rip. Royal Carribean is still kinda new to Alaska, but they have worked out most of the kinks.
I would NOT pay the money for the guided tour. People are so afraid of getting around in Alaska, but seriously folks, there is just 3 real highways. You can't GET lost even if you wanted to!
The cruise is a good chance to see a little bit of a lot, but you cannot appreciate the magnitude and purity of Alaska without going inland and getting in an airplane. It is breathtaking to travel miles and miles with no roads, no towns, no railroad tracks, no farm fields, just land untouched by man.
If it was me....
Fly into Anchorage. Spend 2-3 days here and see the Anchorage Museum of History and Art, The Native Alaska Cultural Center, the Alaska Railroad Station and Ship Creek (to go fishing in downtown Anchorage), the beginning of the Iditarod, the Alaska Zoo, and Chugach National Forest. Eat at Orso, Moose's Tooth, Simon and Seaforts and the Crows Nest atop the Captain Cook. Stay at The Captain Cook (nicer hotel) or any of the bed and breakfasts (less expensive).
Drive north on the Parks Highway towards Mt. McKinley (but do not call it that while you are there, call it Denali). Stay in Talkeetna the first night at the Talkeetna Lodge. Explore the funky town and watch the climbers prepare to go tackle the mountain or recover from their attempts. Take a flight tour of the mountain.
Continue on to the Denali National Park. Remember that the park is the size of the state of Massachusetts, so you want to research what you want to see carefully. Stay at the Denali Princess Lodge and the next day do the bus tour (you cannot drive into the park in your own car, they limit the traffic and make it safer for you and less invasive for the animals by doing the tour on a bus. Its a day long trip.). Stay another night at the Denali Princess Lodge.
Drive back down the Parks highway through Anchorage to Girdwood (day long drive). Drive down on the Seward Highway to Girdwood and spend the night at the Alyeska Prince Hotel. Dinner at the Seven Glaciers (4 star restaurant on the top of the mountain with a view of seven different glaciers). If you don't eat that the Seven Glaciers, the Chair Five for burgers and beer and chatting with the local folks. If you did not get to do the flight tour in Talkeetna then they have some nice ones here.
Drive down the Seward Highway to Seward (try to get there on a non-cruise ship day) and spend the late afternoon and evening wandering around and enjoying the town. Spend the night at the Edgewater inn and then go on one of the Alaska Heritage Tours wildlife cruises with the salmon lunch on Fox Island. Spend another nigt at the Edgewater and then be ready to board your cruise the next day (unless you go out of Whittier, then you will have to go back almost to Girdwood to get to Whittier).
Thats a 7 day trip and it is not close to everything.

I think June is a beautiful time to go as the light is so amazing to people (23+ hours in Denali) or August as the fireweed is in bloom and is stunningly beautiful.
Have fun and let me know if you have any questions. Enjoy!