if you’re planning to do the full hike, I would recommend 2 nights minimum. To maximize your time (and if you’re both very strong hikers), I’d do the full 11 miles all the way to Kalalau, stay the night and the next full day, then 11 on the 3rd day. If you aren’t strong hikers then I’d recommend breaking the hike up and camping at Hanakoa (mile 6). I wasn’t super impressed with the camp site, but I was told there are better areas I didn’t see. The full 11 mile hike in was very grueling (extremely muddy and you are literally climbing up and down boulders at times) and done at the very “quick” pace of 8 hrs (no significant breaks). I really needed a full day there to realize it was magical, not sure I would’ve had that if we had gone back the next day. We were able to do it in about 6.5 hr on the way back.
if you camp you will definitely need to get a permit and make sure you know when reservations open up (maybe a month in advance) and be ready to buy ASAP.
I would pack light (<30). Grab either one light share tents or some hammock tents, sleeping bag liners (ditch the sleeping bags), backpacking meals (so you can avoid bowls and bringing more camp accoutrements than necessary). You absolutely need hiking poles and hiking shoes. Plenty of stuff to make a fire, but may be useful to bring a small camp stove and buy the isopro from surf and snorkel (or something like that) in Hanalei. Bring a Grayl (or cheaper alternative) for water filtering/ purification as you will be getting all your water from streams and creeks. The water is very clear but there are concerns with lepto and some other nasty bugs. There is a gross toilet there. Lots of people keep it stocked with TP but you never know.
camping areas were great and plentiful. Grab a spot close to the waterfall so you can be close to the good part of the beach and the waterfall (shower/water source). Many of the sites already have fire pits and home made chairs, ropes for lights, nails for backpacks.
don’t camp too far onto the beach or too near the cliffs, as there are rouge wave and goat/rock slide concerns.
you can forage some bananas, coconuts, liliquois, guavas, squash etc., but be wary of anything without an outer covering (rat lungworm).
you can’t park at the trailhead (and no cell service), so you’ll need to set up a shuttle. I also don’t think you can park overnight at the shuttle stop so you need to cab there and cab/hitchhike back to your car
I’m sure I’m missing a few things but let me know if you have any additional questions.