Be sure to spend some serious time on cruisecritic dot com's message boards before your cruise! You can find out everything you need to know over there, it's a great place.
You can even get info on your destinations, to see if there are tours you can self-book, instead of going through the cruise ship. For instance, we did a tea at a garden while in Skagway directly through the cruiseship, but when we went to the glacier in Juneau, we just hopped on a local bus to get up there which cost a LOT less. We just had to manage our time really really well.
Tips, photos (some cruiselines are doing a CD now, like photopass), alcohol, soda when not in the fancy dining room (and some cruiselines might charge in the dining room as well), excursions. That's what I remember from our cruise!