We sailed the Zaandam to Alaska in 2002 and loved the ship....although we did think the organ in the atrium was a bit silly... I'm sure you'll enjoy the ship too.
Glad to hear you're going to Glacier Bay.....Awesome!
It's hard to suggest what you can do in ports. What's your budget? What sort of things do you like to do? Any physical reasons that could hold you back?
Juneau.... Make sure you somehow get to see Mendenhall Glacier. Relatively inexpensive on the busses from the dock. If you like wildlife, I highly suggest a whalewatch trip. You'll possibly see whales from the ship but the tours get much much closer. At the dock is the Mt Roberts tramway. If it's clear out you could try that...unless you're afraid of heights. Beautiful views from up top.
Ketchikan.... we got a kick out of the Duck Tour vehicles. Fun and a good way to see the town. We didn't go but I heard a lot of people enjoyed the lumberjack shows and also the Totem parks. There are totems in town too. Go up the funicular and walk back down the Married Mans trail. Very nice and easy.
It's fun too just to walk around the towns. Juneau's a bit hilly as you leave the touristy area.