What shore excursions to take? My first suggestions can be done without buying the excursions on the ship.
Ketchikan
I recommending shopping till you drop. There's plenty of shops next to the pier. If you're willing to spend some money taking a ship sponsored tour, take the totem city bus tour and discover why Ketchikan is considered the totem pole capital in the world.
Juneau
Take the tramway up Mount Roberts. There is a free bus to the tramway terminal. Then walk and walk and walk along the trails. The scenery is awsome. Take a lunch with you to delay coming down the mountain on the tramway. If you're willing to spend some money on a ship sponsored tour, take any of the tours to Mendenhall Glacier. Another option is your best chance to see whales on your trip up close, take the jet boat whale watching tour.
Skagway
Take the White Pass & Yukon train. Again, the scenery is awsome, so is the history of the train. You can buy tickets at the station and save some money buying the tickets on the ship. It's not that more expensive to buy the train tickets on the ship. If you're willing to spend more, a ship sponsored tour, take one of the one way train tours. You take a different form of transportation up the pass, and take the train down.
Wrangel
Wrangel is a small fishing-timber town. There's not much to do in town without taking a tour. Now's the time to go fishing from this fishing village, or your best chance to see bears.
Try either jetboat Alaskan tours fishing boats or the Anan Wildlife (Bear) Observatory tour.
There's just too many things to do at each port, the big reason why many will take another Alaskan cruise.
