I, too, made a trip to Hawaii when my oldest was 18 months old. It was...interestingly fun :)
Flight: Toys, snacks, snacks and more snacks and a couple DVDs wouldn't hurt either (also a blanket and some cozy stuffed animals, if he has a few favorites). I never used Benedryl--I prefer not to unnecessarily medicate my kids. We did have him in his own seat with his carseat, so he was at least sitting in a familiar environment. At naptime, we tried to follow as similar a routine as we did at home (5-10 minutes of DVD + some walking around while being held). He did nap maybe twice for maybe an hour at a time--which is pretty good for the circumstances! However, he was exhausted by the time we got there. There may definitely be some crying/sobbing/screaming stints. Walk the plane. The flight attendants are usually very understanding about this and we ended up hanging out at the back of the plane a few times (so as to least disturb the rest of the passengers). Also make sure you have more diapers/wipes than you think you'll need! For both Oahu and Kauai, there are Walmarts close to the airport (or at least on the way to the hotels, as the case is for Oahu), so you can pick up the rest of the diapers, wipes, milk, etc you'll need for your vacation--no need to bring EVERYTHING!!
Jet lag: do your best to slip right into your normal routine as quickly as possible. That may mean keeping him awake (as much as he may be exhausted) for an extra hour or two so that he goes to sleep closer to his normal clock-time (vs body-time). Either that or you'll have a kid going to sleep at 4pm and waking up at 3am!! I remember one time we were out there again when my boy was 4-yrs old and he fell asleep at the restaurant we were at for dinner--he put his head down literally ON his plate (thankfully, we hadn't been served food yet!) and was konked out until morning!! But since he did make it to morning, he was able to flip into his normal schedule times--score!
Another tip I learned is to ship things back home. So, if you have diapers, clothes, toys, non-perishable food, souveniers, etc left over that you don't want to pack for the trip home, head to your hotel's business center or the nearest FedEx office (there are a few) and ship it home! At the current baggage fee rates, it'll be close in cost but FAR more convenient to not have to lug all the extra stuff through the airport there and then from ORD or Midway home!
But most of all...HAVE FUN!!! I'm so jealous!! We were supposed to go back this year, but had to back out due to finances and I'm thoroughly looking forward to when we have all that cleared up so we can go again. Hawaii (particularly Kauai) is my favorite place on the planet!