We took our 15 month old ds to WDW April 2001. This is what we did:
Pre-booked our seats to get the bulkhead (more leg room and a wall in front to you)
Requested a bassinette that attached to the wall in front of us for the baby to rest (he's too big to sleep in it but I got to eat an entire meal while he played in it on the way to WDW)
Brought his favourite snacks ~ he eat 11 digestive cookies on the way to WDW. I'm bad, but it made EVERYONE happy!
Brought NEW toys for him to play with.
He was GREAT on the way to WDW, he was a bit restless on the way home.
On the way home, we spent a little more time playing on the floor (again the bulkhead) and walked the aisles a couple times. Again, I am bad, but when the bathroom wasn't busy, we played in with the sink for about 5 minutes a couple times.