The Diono Radian car seat is by far a favorite for us when we take our little guy abroad. This picture says it all! Compact and convenient. It allows our arms to be free to wrangle the kids.

It folds flat and coverts into a backpack, no extra bulky bag needed. We can set it up on the plane in a matter of minutes and our toddler sleeps like a baby in it.
Bonus: These seats are narrow so they don’t take up a ton of room. So feel free to rent that compact rental car and save a few bucks.