That totally depends on what you want to do and when you're there, I would say. I booked everything a month or so ago, for our trip in a few weeks. I wanted to sync up timing of some things. Not once was I told "that's booked up" or anything of the sort, and everything had a 24-48h no-fee cancellation policy, so it's also easy enough to cancel stuff. For larger boat tours, etc, I think you would be just fine waiting until you get there, and if you're flexible on dates/times - probably even the smaller group tours. If you know what day you want to go to Pearl Harbor, you'll definitely want to get your USS Arizona tix ordered in advance, b/c even doing that shortly after the window opened for our date, my options for the time of day were extremely limited. The only other thing to consider is booking water-related things for early in your trip, in case they get canceled for weather conditions and have to be rescheduled; this mattered to us b/c a boat ride was my son's biggest request, but otherwise I probably wouldn't have cared that much.