Lightweight harness seat?
I second the safe rider vest. We got one for our almost 5 year old and it worked out great. It's light and convenient to pack in a carry on. No worries about lugging a car seat through the airport, or worrying about damaging your seat by checking it. Another plus is that the rider vest doesn't expire, so you can use it for other kids as well.
It's pretty easy to get in and out of. Maybe practice a couple times in your regular car. We just really played it up for DS that it was his special car seat vest he only gets to use on special occasions. He liked it.
Just for cars. I don't worry about the plane with older kids. The seatbelt is going to restrain just fine in turbulence. In a crash it could potentially be iffy, but so few crashes are survivable and statistically so unlikely (safer than a car) that it's an area I don't worry about.