I'm 63, from Texas, in good health, my wife younger and in great health. We traveled with 3 couples but all agreed to "do our own thing" from the beginning. That was important. Here's some things we wanted to share:
First, the obvious: Prepare and plan early. Pack for cold and wet (gloves, scarf, headgear) but layered for the wonderful summer warm (60-70 degrees F) we enjoyed; Mid-July is a great time for this cruise. We had 9 sunny days out of 10, but note: that is very very unusual. Pack casual wear, pretty cool for sandals or shorts, altho we spent one afternoon sunning on the aft deck. Carnival has a great program to forward your baggage to the Sea-Tac airport at end of cruise ($20 per) - do it! (Then you can go baggage-free to see downtown Seattle and take the 3rd Avenue $2.75 metro rail to the airport.)
Get a balcony cabin (cabins on decks 6, 7, and 8 are all the same). Take binoculars (our 10x25s were fine) and a good camera (long long distance shots got us eagles, two bears and a Sitka blacktail deer): this cruise is about scenery, scenery, scenery. Spending time in the casino or bars misses the whole point of this trip: we took 817 photos.
Board early. In fact, come to Seattle early - it's a great city for tourists, plenty of charming, good-value downtown hotels (the Vintage Park was ours). See the marketplace, Pioneer Square, Purple Bar & Grill; take the ferry or the water taxi. Be prepared for steep San Fran-style hill-walking...seriously.
As always, your being friendly to the cruise staff (all of them, from Minute One on board) will be returned ten-fold. Remember: this is not an exclusive resort where everything must be perfect. It's a cruise ship with 3,000 people: keep your expectations at a proper level, you'll enjoy your vacation more.
Breakfast in the dining room is quieter and with better service than the Lido deck grill/buffet. We try all of the eating spots on our cruises, and we generally enjoy the food, but these are not 5-star eateries!
The excursions are more expensive than in the Carib, but some are worth every penny. In Skagway, take the train. In Juneau ride the tram or whale-watch, eat at Tracy's King Crab Shack. In Ketchikan (our favorite port of call) get past the cruise shops & don't miss Creek Street, eat at Halibut Hole; we saw eagles everywhere there. Walk 10 blocks to downtown Victoria, enjoy the Inner Harbor.
Note: In the summer, sunrise is 5 am, sunset 11 pm.
As a Bucket List trip, and with the weather we had, we wouldn't change a thing. It's back to the Caribbean for us next, but we'll never forget Alaska.