My wife and I just did the Kalalau trail in Na Pali State park and I wanted to put out some of my general thoughts to help make it easier for someone else who may be interested. Prior to departure I had a hard time determining what was important and what to trust based on what I read online. Hopefully this is helpful, although your milage may vary.
The Hawaii State Parks office is in what might be the tallest building in Lihue. They will give you an informational brochure with a map and try to sell you a topo map of the entire island. We thoguht this would be a good informational stop but I think best to skip stopping here and use the map in your guidebook.
The trail itself is difficult, but not quite as dramatic as most of the literature makes it out to be. The first half to Hanakoa Valley is the most up and down and felt like the hardest part in both directions. The second half feels much easier. Hanakoa Valley is a great place to stay if you are not feeling up to doing the trek in one day. Plus, staying here gives you a full afternoon at the beach as opposed to getting in late in the day. There is a pretty steep and exposed part around mile 8 or 9 that I was glad to walk across when I was properly rested.
We brought packs and a tent and rented a stove and bought fuel at Pedal n’ Paddle in Hanalei which also sold us some of the other things we forget (such as a cooking pot). There is a grocery store next door that is pretty well stocked. You may be able to save a few bucks by planning in advance and shopping elsewhere.
We found the mosquitos were a nuisance, but nothing intolerable and the seemed much smaller and less vigorous than those we have on the mainland. Bug spray was nice but not essential.
We left our car at Ha’ena State Park. There is not a lot of parking at the end of the road and we saw some cars that were ticketed for being illegally parked. The earlier you can get on the trail the better, but you might get lucky. The guidebooks talk about break-ins at Ke’e Beach and while we did not see this, Ha’ena has better visibility.
We filtered all our water. The guidebooks warn about Leptospirosis and while I am not sure how prevalent this is better to be on the safe side.
There are a lot of long term campers there but there are also plenty of spots.
Otherwise, the hike was a lot of fun and I totally recommend it!