Ride Safer Travel Vest is my FAVORITE for travel. It's a wearable carseat. I've used it for my 2 oldest for a few years now when traveling, nothing better than having your car seat in your backpack through the airport! http://www.safetrafficsystem.com/
If you want an actual seat, Evenflo Maestro, if your child has enought growth weight wise (it goes to 50lbs). You will need another booster later (it's a short booster), but at that point you'd probably be fine with a backless booster for travel.
The RSTV is technically a belt positioning "booster." But it has several features which make it useful for kids that are only just barely within booster range for age. It has an optional crotch strap to help prevent submarining for smaller kids. It has a top tether attachment which literally hold the child's shoulders in place - this is optional to use if you have a lap/shoulder belt and required if you have a lap belt only. For a child who is not mature enough to use a regular booster (which would be any 3 year old) I would always use the top tether attachment. Although since the belt guides are actually ON the child, when the child moves and squirms the seat belt moves with the child. I would feel more comfortable with the RSTV for a younger/squirmier child than I would feel with a traditional belt positioning booster.
It passes all the NHTSA testing standards for booster seats. I would normally recommend it for kids ages 4 and up, but it does have a minimum age requirement of 3 and if a kid is almost 4 and didn't have any other great options for vacation I would consider it. It does have some great advantages in terms of portability.
You want to know the best thing about traveling with the RSTV? Your child actually carries it in his/her carry-on so you don't have to worry about it! Because, IMO, carrying car seats through the airport is another level of hell...
I'm actually looking forward to the day kid2 is in a RSTV so we only have to lug one seat through TSA. It really is one of the best inventions ever.