Not a lot to do in town other than shop, but there's a gondola that takes you to the top of the mountain overlooking the harbor for a spectacular view. Even at ground level, the view of the bay, with floatplanes landing on and taking off fro the water and the forested mountains in the background, is gorgeous. It's also a good place to have a quick bite and sample the beers of the Alaskan Brewing Co., which are pretty good.
Shopping, shopping and more shopping. Jewelry stores everywhere. most of them run by folks who have chains of jewelry shops in various ports on the cruise circuit and follow the ships as the seasons change. Also, lots of amphibious vehicles to take visitors on tours around town and in the harbor. If your grandfather served in World War 2, he knew them as DUKWs, or "Ducks."
On a good-weather day, you'll want to be up on deck with your camera for the 4pm departure. In fact, you'll want your camera handy throughout this cruise. The combination of Elliott Bay and the Seattle skyline will have you making your own postcards, and
Seattle has two cruise ports, Pier 66 and Pier 91. The latter pier has an industrial, non-cruise-friendly vibe to it and is by far the most distant from the downtown/waterfront, so if you arrive early, don't plan on killing time along the waterfront before boarding. If you do decide to grab a cab downtown, Pike Place Market or the waterfront, the port does have a concierge service that will stash you bags for you until it's time to board -- 730am to 330pm on sailing days only. Cost: $3 per bag. It's a great service and more cruise ports should have it.
On your return at the end of your cruise, check-out is a breeze, and tour buses and taxis will be waiting close by as you walk out. Those who recall Carnival's slow, chaotic debarkation procedures of years past will be gratified at the changes.
The smallest of the Alaska cruise ports also is the one in which you'll be spending by far the most time, arriving at 7 am and departing at 9pm. How small is Skagway? Depending on the time of year and the state of the economy, the arrival of your cruise ship will automatically double, triple, or quadruple the population. On the other hand, you will able to honestly say that you fully experienced Skagway, because you'll have enough time to cover every single block of it.
If the ship docks by the railroad tracks, you'll be about a half-mile from town. There are shuttles available, but take the hike in at least one direction. Your reward will be to see salmon up close, staging in a swift-flowing creek, preparing to swim upstream to spawn.