You will love your Alaska cruise. We were there a number of years ago and found the scenery breathtaking.

Regarding the ship, Celebrity's Infinity, we just returned from a Caribbean cruise on its sister ship, the Constellation (same layout - different name). The ship is fairly large but isn't overwhelming as some other cruise ships. Also, Celebrity doesn't cram in as many bodies as possible so you get a feeling of spaciousness. We never had any difficulty finding a deck chair in spite of the fact that the ship was full and the weather was beautiful. That is definitely not the case with some of the other cruise lines such as Princess.

As a bonus, the food was great.

As far as excursions are concerned, I tend to buy them directly from the cruise line since they are responsible for getting you back to the ship in time. In my humble opinion, the peace of mind that it gives is worth the difference in price.

Whatever you decide, have a great time.

Minna