Last trip with DS (13 months) I brought many things. I made sure I had a wide variety, and still, you never know what will happen.
I try to find some "new" toys for him - which may be old favorites that have been put away for several weeks before the trip - or new things. I try to bring out toys over time, so he does not see all at once and get bored. On one recent trip, I had a new soft book with a dog bone attached. He spent quite a while happily putting the dog bone in the "Water" in the book.
I also make sure I have lots of different snacks. I have a snack trap for him (from babies r us). I'll fill it with Cheerios or small crackers, and it may occupy him for 30 minutes at a time. Sometimes, just a small bag of cookies is enough to occupy him for a while - just because he likes the crinkly bag.
Then - I make sure I have plenty of beverages - milk, water, juice - in both bottles and sippy cups. I make sure I have as many options as possible to get him to drink during take off and landing.
Still - sometimes it goes well and it a very easy flight, and sometimes, it is a disaster. Most often, it does smoothly. I always buy DS his own seat - but that does not always help. On last trip to WDW, flight attendant asked me to take him out of car seat and hold him because he was screaming so much during landing. Thankfully, it was only for about 30 min or so (but it seemed like hours). However, more than 24 hrs travelling to Athens, DS cryed for less than 10 min the entire trip.
Good luck.