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.
We got the Evenflo SureRide for an occasional/travel seat when my DD outgrew her Scenera. It is very lightweight, easy to install (in our car at least) and a lot more cushy than the Scenera which I wasn't expecting for the low price (seems to go on sale at least here often so if you're not in a hurry I'd keep an eye out). I was looking at the Maestro also but the SureRide seemed like a better match for our situation. The Maestro does have the benefit of converting to a booster but it only fits kids harnessed up to 50 lbs and 50 inches whereas the SureRide goes forward facing up to 65 lbs and 54 inches (but does not have booster capabilities, though you couldn't use a booster on a plane anyway so you'd need to check it).
And I just saw the age of the thread... oops! Well I'll leave it here anyway since I typed it all out.