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. Tours that have limited space should be reserved ahead of time. Activities that are offered several times daily can probably be booked closer to travel.
Check with the tour company when you inquire for information about their reservation policies.