We flew Delta but I know that they checked the car seat and stroller for free and didn't count as luggage. You might want to call the airline or check their website for their policy. And I'd check on the pack 'n' play. Because with all the new fees, if it counts as a carry on, you may have to pay for your diaper bag, purse or whatever else you are carrying on board.
I packed 1 diaper for every hour of travel (including layovers), a few extra onesies, some of her favorite snacks, and a couple of small toys she liked. She didn't take a pacifier and was breast fed but I did take a playtex drop in bottle, a few liners and some formula just in case I couldn't feed her when she needed it.
Things went well and smooth for us, hope it goes as well for you.