Since DD moved out of her into her Monterey, I'm down an "extra" booster. I have a backless Grace TB that I use when driving another child and there's a BubbleBum I have available, but prefer to bring only for our own emergency travel needs. I want to get one more backless booster to use when I have a third child in the car with me. This would usually be a third grader (so 8 or 9 year old). What's the best seat to get for these somewhat larger kids?

ETA: I forgot to mention that we're in a Sienna, so the "third kid" would be in the third row...but there are no other carseats to contend with.