We have used a safeline car seat/stroller on several trips - since our daughter was one - she is now four and moving up to a booster seat - but the car seat/stroller lasted through three summers of vacations - it is great on the plane (FAA approved - we've only had show a stewardess once - the approval notice is right on the seat - there's nothing to gate check) and it's a stroller (and/or carries stuff if toddler wants to walk).
I think the Right Start chain sells them but I bought it online through safeline.