The Highway to Hana gets near top billing in every guidebook, and they have plenty of detail on where to stop and how to plan your day so I’ll defer to them.
What you need to know from a windsurfing point of view is that this is a whole day trip, so plan it for a light wind day, or a rest day if you can! When the trades a lighter it probably means less rain as well which should make for a better trip.
Despite being 35-55 miles depending on where you start from, it’s a long drive with the majority of the trip is at a speed limit of 15mph due to the narrow winding roads you’ll be driving on – we counted 30 corners in a mile, and there are over 50 one lane bridges. For the driver this requires a lot of concentration so it’s easy to miss out on the views and spend all day trying to squeeze your van though narrow gaps. If you can share the driving and get a few people together that’s the way to go.
Be warned that everyone seems to set out between 8am and 9am, and parking spots at the various attractions are minimal, so you might not be able to stop everywhere you want. It’s kind of like a traffic jam but with amazing views. But if you’re coming back the same day you’ll find the same spots virtually empty in the afternoon, so you can plan on a visit then. Or if you’re staying over in Hana it might even be better to set out a little later.
On a practical note – make sure to bring mosquito repellent if you’re the type to get bitten, and some shoes or hiking boots you don’t mind getting muddy and a bit wet, since you’ll be doing at least a little bit of clamouring up wet boulders and muddy paths to get to some of the better spots.