I'm a former Flight Attendant and I fly a lot of long haul trips with my three kids.
I found this to be the most difficult age. They move a lot and are hard to reason with (no bribing!)
Yes, your child will be better off in his own seat. If you didn't book him a seat, find out if the flight is full. Ask nicely at check-in if there is an extra place, if it can be next to you. If you are bringing the car seat, you are allowed to use it on board if you get the extra seat (either by a window or in the center seat of the center section if there are aisles on both sides). Ask even if you are not bringing the car seat. Just a reminder to NEVER check a car seat in as luggage. Always bring it to the gate and if you are unsuccessful, have it gate-checked. The airlines lose and break seats all the time.
Some parent buy new presents for flights. Some hide proven favorites a few days before leaving. Right as the plane is leaving the gate, everyone has to be seated (in your lap or in her seat regardless). This is really hard for toddlers so be ready with a new toy just at that moment. Hopefully, it'll keep him occupied until you're in the air. Remember, no electronical gadgets during that time!
Select toys that don't make ANY noise. Even a quiet noise is irritating played over and over. I found simple things to work best; coloring toys, play dough, stickers, books with flaps that open, small plastic animals and cars, etc. Nothing that can't be lost. Bring his second favorite stuffed animal (his favorite goes in the checked bag or stays home).
I don't want to talk about electronical products in too much detail, but for this age, unless you have something or can borrow it for the trip, I personally think he's too young for it to really be worth the fuss. But you know your child best! Be sure though that he'll either watch it without sound or use the headphones. Practice at home first!
Please don't fret his ears. It's a myth that children are supposed to suck on take-off and landing. The AAP recommends that the child simply be awake about an hour before landing. Just moving his head, laughing, coughing, talking will open his ears. Offer him something to drink but don't force. On take-off, many children fall asleep on the tarmac and I hated seeing parents waking them unnecessarily and forcing them to drink. Let your child sleep!
I never used any drugs to get my children to sleep but if you're thinking about it, be sure to discuss it with your doctor. Many children have allergies to them and twice on flights, I saw children get the *opposite* reaction. Also, the child can get disoriented and dehydrated with them. If you have a super-sensitive or hyperactive child who will have problems sleeping, look into this. Otherwise, I would give it a miss.
If you have a good quality baby carrier, be sure to bring it. Flimsy front packs like the Bjorn are long outgrown but the better ones go to age 2 or 3. Something like an Ergo, Beco or Mei Tai is super-useful in the airport and they're magic for calming babies during the flight. Even the most independant toddlers can get clingy and want to be held in a strange place like an airport, and he will find the WORST time to get this way (like when you have 5 minutes to get to the gate!) Just pop him in and keep going...
Use a backpack or backpack-style diaper bag. Pack your purse and for yourself, use a "bum bag" or tiny purse around your neck with the essentials; wallet, cell phone, tickets, a few personal items... Get a bigger backpack. Mine goes to my knees when placed on the floor. Pack more diapers than you think you'll need, put in plastic bags and wrap with rubber bands to save space. Change of clothes for each of you (maybe two for her). For a night flight, just have her fly in PJ's.
Learn to change him standing up and use the handicapped lav on board for more room. He's too big for the airplane baby changers.
Bring a bungee cord to double secure the stroller before leaving it at the gate. Bag your stroller if you want with the bungee cord over it but make sure the gate-check tag can be seen.
-Bring an empty sippy cup to fill during the services. This will prevent spills in the air.
-Get up and move around with him but keep an eye out. Those carts the F/A's use are the perfect height for hiding little ones
-Bring snacks. Don't fret security. I've never had anything taken away (yes, water but not snacks) and that's the only risk. If you have something not allowed, they simply remove the item. No scenes, no speeches, no arrests!
-Try to sit in a bulkhead seat so that there's no one in front of you
-Bring slippers, especially if you're not in the bulkhead!
For more information, I wrote an article on this subject about 10 years ago. It's based on both my personal and professional experience of flying with kids. It's totally non-commercial and other parents have contributed over the years;
http://flyingwithchildren1.blogspot.com
Hawaii is great for kids. Don't spend much time in Wakiki and enjoy one of the most beautiful places in the world!