Book the Na Pali coast boat tour before you arrive rather than after - the reputable operators fill up weeks in advance in peak season and the difference between a good operator and a crowded budget one is significant enough to matter. The north shore sea conditions also change with the season, summer is calmer and the tours run more consistently, winter swells can cancel departures and the good operators will tell you this honestly while the less good ones won't. Ask the inn staff which company they'd send their own family with - that's the right question.
Hanalei farmers market on Wednesday morning is worth the early start - the vendors there are actual north shore farmers and food producers not a tourist market, the prices are what locals pay, and the fruit situation on Kauai's north shore specifically involves varieties that don't make it to the mainland in any recognisable form. Go early, bring cash, buy more than you think you'll eat because you'll eat it.
Rain on the north shore comes and goes fast and the mistake is letting it change your plans too dramatically - kinda the wrong instinct to retreat indoors when it starts because it usually stops within an hour and the landscape after rain on the north shore is the landscape that the photographs you've seen were taken in. Waterproof layer in the bag, carry on. The days when the rain is genuinely all-day are fewer than the reputation suggests and the inn staff can read the weather patterns better than any app.
Sunrise on the east-facing coast near the inn is the right way to start a day and most guests who do it once start doing it every morning - the light on the water at that hour, the mountains behind Hanalei catching the first colour, nobody else around yet. It doesn't require hiking or planning or anything more than being awake slightly earlier than feels natural for the first day. After that it's not a discipline, it's just what you do.
The drive to the end of the road at Ke'e Beach in the late afternoon, after the day-trippers have gone back, is a different experience from the same drive at 10am - quieter, lower light, the mountains above the Na Pali already going dark while the sea is still bright, and the feeling at the trailhead of being at the edge of something that doesn't continue in any direction you can follow. Go at least once in the late afternoon. Because that's when it's really itself.