You will need to make as many pre-arrangements as possible for a trip to Alaska. Hotel and rental cars can be expensive and hard to find if you wait too long. So you'll definitely want to reserve these ahead of time. On tours, you will want to use sensible judgement. If the tour is exclusive or has limited space, you should reserve ahead of time. If it is an activity that happens several times daily, you can probably wait longer. Check with the tour company when you inquire for information about their reservation policies.