As part of the pre-travel research (and I always do this), I tried to determine if it would be better to pre-book some activities. Pre-booking is good when you're short on time, when there's a possibility that excursions will be sold out and/or if you want to have some structure to your trip. I've also found that trip structure is key when traveling in groups larger than two and people who have differing opinions on timeliness, etc.