We've did the cruise/tour the last week of June, first week of July. Weather was perfect but from what I've read that varies from year to year. Related suggestion-do the land tour first so you can have the cruise to rest.
It is something you'll remember forever.
June and July are definitely the best months to visit Alaska. The last couple of weeks of July it sometimes gets a little rainy. But Norma is right, the weather does vary from year to year, so you can't really count on anything.
Unfortunately to see the Northern Lights, you need to come up in the winter when its dark (and put up with the cold). During the summer months its light out at night and you can't see them.
I'm sure you'll have a fabulous time!