Also, to let you know, that car seats without harnesses are not FAA approved, and therefore can't be used on board. I mention this because I noticed justthat posted about cheap, lightweight car seats to carry on back and so I'm not sure if she is referring to regular car seats, or those travel ones that I've seen on the Net, but they aren't certified for use on board planes. They aren't certified for use where I live in Canada either - our laws are all kids from 40-80lbs, must be in a booster. We have harnessed boosters for kids who are almost 40lbs., because putting kids in regular car seat belts between 30 - 40lbs., is dangerous especially if they are smallish, as well as against the law in my country, yet it's mostly allowed in your country - this causes travellers so much confusion!
It's very confusing to see all of the different car seat laws between the different states and Canada, so make sure you really understand the Florida law for car seats while you are travelling down there. The most confusing part is the use of ages - ages have nothing to do with safety crash tests, plus, they still use old average age weights for these regulations, so for me as a parent travelling to Florida, I know that what I'm used to for car seat safety in my country is better than Florida law, so my daughter will be very safe while driving in her harnessed booster at age 3, but I can totally understand why people get confused by what car seats to bring - good luck and I hope you have a very safe trip.
Tiger