Need a new carseat

I just realized that my 5yo is too big for the Evenflo Tribute we use as a spare in DH's car. What's a good option for us? It's only used occasionally, so cheap is preferred and something that converts from 5 pt harness (which he still needs) to backless booster would be ideal. I also would like something really light as we'll be hauling it down to Disney in Feb and possibly other air trips in the future.

Need a new carseat

Graco tranzitions

http://csftl.org/graco-tranzitions-review/