I have only done one Alaska Cruise, and it was the itinerary that included Endicott Arm. But I get to AK fairly often for work (72 days this year) and have seen a lot of the interior between Anchorage, Valdez, and Circle. The cruise, which was in July, was disappointing to a lot of the folks on the ship because it rained most of the time. Knowing a bit about the weather my family was prepared and we had a good time.
If I had to do it again I'd fly into Anchorage and take the one way trip south. First because I could take some time to see more of Alaska, and second because it would end in Vancouver, a city I enjoy visiting. I'd plan on two weeks, with two days spent taking the local ferry from Whittier to Valdez and back. You'll see more wildlife and glaciers on those two days than on the entire trip down the Inland Passage.
I'll offer the following suggestions:
Be prepared for rain. If it doesn't happen, great; but it usually rains, and people up in this part of the world don't stop going outside when it does.
Get up early. The time of year you're looking at it's daylight 20+ hours a day. I can guarantee if you're on deck at 5am you'll be almost alone and you'll see the wildlife everyone else will miss.
If you're planning on taking pictures get a really good telephoto lens. 300mm minimum. A cell phone will not do justice to your memories.
Pay attention to the ship schedule. You can't do much if they dock in Victoria (which they have to do by international law) at 7pm. It's not worth standing in line to get off (and back on) for that.
Be patient. It takes a really long time to get on, and off, one of the cruise ships. You'll spend an hour snaking the line off and another standing on the dock waiting to get back on. Plan accordingly.
Unless there's something you just can't do otherwise, don't use the planned adventures. They are expensive, and you can often do better on your own. However, you're going to be competing with 10,000+ other tourists for the same attractions at every stop.
Take the train ride up the pass out of Skagway. Pack a lunch. It's the best thing to do there (by a huge margin) and incredible scenery.
If there's an itinerary that goes to Sitka, seriously consider it.
Take a good book, make friends with the staff, and sit back and enjoy. It's a long slow ride.
Last, it's illegal to maim unsupervised (or poorly trained) children (or their parents). Throwing them overboard is also frowned on. If you have issues with these types it's best to look for an all adult cruise.