DD will be 3 in March and is just about 40 lbs. Not sure of height as she's asleep at the moment but she is fairly tall for her age and wears size 4T clothes. We are using a Radian as our main seat and our backup (grandma's car) and travel seat has been a Scenera. Well, it only goes up to 40 lbs so we'll need something else. Not necessarily for grandma's car as we can arrange for her to just go in our car with the main seat but we will need something for travel.
What can you suggest that will be not too expensive and nice and light to carry? We are expecting baby #2 in April so portability will be especially important for travelling with two little ones. And I don't want to spend too much since it will only get used a few times a year probably.
A ride safer travel vest is our favorite travel seat for that age. It can't be used on a plane (though at 40lbs the lap belt is fine on a plane). But it fits very easily in a suitcase. A bit cumbersome for every day use, but for traveling worth the tiny bit of extra effort to buckle to not have to lug another seat through the airport. We've been using ours for close to 6 years! My youngest is finally large enough for it and we're so excited to not have to take any actual harnessed seats with us when we next fly! http://safetrafficsystem.com/ver4/
Since you have a Radian, have you considered traveling with that? It folds and you can get a carry case or carry straps and tote it as a backpack. With her being so young, I would want to keep her in a seat if possible.
I do love my Radian but that thing is heavy! It's nice that it folds and has straps but there's only so many heavy things I can carry through an airport. I normally have a backpack on anyway with DD's things so that I have my hands free to tend to her.
We did end up ordering the Evenflo SureRide when it was on sale after the holidays for $70. It's nice and light and was easy to install FF in our car, even beside the Radian. I like that it's tall because DD has two more shoulder slots left when FF as opposed to only one in the Radian (she's fairly tall). The only drawback is that is can only RF to 40 lbs and it has fairly high sides on the seat. Still, I'll have to try it out cause right now I'm think we may even make it her permanent seat in our car...
I have lugged a radian through the airport. It wasn't too bad when I only had 2 kids and I could bunji the radian to the back of the stroller. But carrying it while pulling a carry on suitcase or pushing a stroller, ugh, no way!
A lightweight seat is not too bad. I find it more awkward than anything. You can attach it with a luggage strap to a rolling carry on bag and roll it through the airport if that makes it easier.
Last fall we traveled to Japan sans car seats because we didn't need them there at all (public transportation is awesome!) and it was pure heaven to not have to lug any seats through the airport!