We use a five point harness car seat for our 4.5 year old who now weighs 42 pounds. So, on our recent trip, we thought we would try a booster seat. I found the Bubble Bum on amazon and bought one (I actually checked out their website as well- turns out it's made from the stuff of life jackets).
It was AMAZING to fly with one car seat (little bro) and to have our older son's seat deflated and rolled up in our luggage. How easy to get through the airport!!
Once we got to the rental car, it blew right up and had all the right clips to make the car's seatbelt adjust to the right places on our (bigger) little guy. What a great invention!