First time we flew with DD she had just turned 2. We took a sit n stroll (car seat, becomes a stroller). It was nice to not have a car seat & a stroller to lug around, but DH got stuck with the job of folding up the wheels and carrying it onto the plane (with sleeping child IN it), he didn't appreciate that. And the thing was like $220. He doesn't want to use it when we go to Disney this summer, so I may just sell it. I liked it because we went to Jamaica and I felt much better with her being in a car seat on our crazy taxi rides.
This time we are going without a car seat. I just hope she stays seated, the car seat with the shoulder straps really keeps her from wiggling out...a plain old lap belt can't do that.
Our daughter did fine on the "big" plane, only one truly bad moment, when we had to turn off Finding Nemo (we took a portable dvd player) in order to land...she cried for the 20 minutes or so it took us to finally land. (In constrast, she screamed her head off on the little Cessna flight from MoBay to Negril).
Take plenty of snacks and a few things to keep your child busy (a magna doodle, a portable dvd player, a color book, even stickers (to stick on seats and pull off and stick on again) entertain kids for a long time.