Portable/light carseat for 2 year old


So my sitter has been using the "baby" carseat for my 1.5 year old but I'm about to pop out my 2nd and will need that carseat back for the new baby. What Is a good, portable, yet not super expensive carseat that she can use for my 2 year old? She would be removing it from her car regularly when he is not with her so I prefer something that is easily inserted to her car.