when my son was 18 mo old, me and him flew to europe to see my family. we spent total of about 17 hrs on the planes and then also some in layovers. yes, i second the advice of trying to not let him nap while you are waiting. i literally had my son run from one end of the terminal to the other "chase you to the escalator!!!!" we wondered around and explored, and when we got on the planes he'd usually fall asleep at least for a little bit.
depending on your child's interests, you can bring some entertainment on the plane. i had some of his fav books, and a couple of new ones, he likes to read. he wasnt a whole lot interested in the toys. don't bring too much though, it will just create extra mess.
a friend of mine suggested bringing some paper clips - to make chains and bracelets and what not, it sounded like and awesome idea - even if you lose some, who cares, but my son wasnt really into it that much, maybe yours will?.
i had a few treats, too for emergencies, which did come up.
i didnt bring a car seat. i had a stroller and a bag with wheels. i 'hooked' the long handle of the bag with wheels to the stroller, so when i pushed the stroller, the bag wheeled too. when i carried my son in the carrier, i put the bag in the stroller and would push it that way.
change of clothes is a good idea, for both of you actually. i had to change my clothes twice because he spilled stuff on me all the time.
i had some cartoons on my phone. if you do decide to do that too, choose the ones where sounds and speech dont matter a whole lot. because of how loud the plane is, you cant hear it on the phone, so my son would get really irritated, and he didnt want anything to do with headphones (plus, you dont want to disturb other people), so he ended up only watching a couple where everything was clear without the sound.
good luck with the flight!!!
