Baby monitor if you use one.... white noise machine, too?
IDK how old your baby is, but I've always found that traveling with an infant car seat is quite easy. I trust my own more than one from a car rental company. If you don't want to carry it on the plane, you just check it with your luggage. However, if it is not a full flight, pretty sure you are allowed by law to put the infant seat in an empty seat (next to you, of course!) without paying for an extra fare. I always wear my LO when flying, but it is really nice to have somewhere to put them down if they fall asleep and you just want to read a magazine, file your nails, enjoy a solo moment, etc.
Good luck! Travel with babies is really easy!