I was just thinking back on our last trip and I thought I'd post about something that's been extremely helpful for me over the past few years--CARES harnesses!
We usually take advantage of Magical Express, and therefore, don't need car seats to transport our two small children. We also don't want to take their carseat and booster seat just for two 2.5 hour plane trips. So after a bit of research, I found CARES harnesses, and boy have they been a Godsend!
CARES harnesses are FAA approved/certified, so they are extremely safe, and are designed for children 22-44 lbs. It's also very small and lightweight, which allows you to pack lightly--it comes with/fits in a small sack and only weighs about a pound. The front part of the harness is very similar to those on car seats, so your child will be comfortable and used to the fit. I have to admit, when I first saw them I was a little intimidated about the installation, but they also come with instructions, on both paper and a DVD, which make it easy.
They are a bit on the pricey side, though. So purchasing one might only be worth your money if you travel with your small children on a regular basis, and have a carseat/booster waiting for you at your destination. But if you do an Ebay search you can find several reliable vendors (I can recommend one - PM me!) that rent them out out for a nominal fee.
CARES' official website.