You want closed toed shoes and something that laces or straps good and tight, If it's muddy it will pull (suck) the shoes off your feet during the hike if they are not tight or something you can tighten, but you also want a shoe you can swim in (something like a good canvas shoe with a good sole, converse style either low or high top should be fine) keep in mind the soil WILL stain them reddish/orange "permanently". We wore shorts and T's and just dried out on the kayak trip back, you could a swim suit under your shorts, "but its a wet trip anyway", take a towel to put in your dry bag and a bag lunch or granola bars or trail snacks to re-energize at the falls after your swim before the trip back, you will take about a 45-60 min break at the falls. YES make your reservations in-advance and NO it shouldn't be to early to make them - you might want to ask for reservations on a day there isn't a cruise ship in port, as that can delay your your kayak departure, as well as being a part of a much larger group and losing some of the more personal aspects of the journey, but regardless it is well worth it rather your group is large or small! OH - also a double seal freezer bag would be best to keep lunch/snacks and camera/phone in - you will have a chance to pull out your camera/phone for pictures along the way - "and you will want too"
Hope this helps
Take time to enjoy the Island - try not to overwhelm yourselves with to many adventures - but don't miss this one, Waimea Canyon (self guided if you can), or Napali Coast sunset dinner cruise (about 75.00 to 125.00 USD per person and I don't recommend booking it until you get to the Island - you will get A MUCH BETTER DEAL)
Rick