We have a combo seat for our 3.5 yo who didn't outgrow his convertible...but handed it down to his younger bro (he outgrew it RF, though so we went with a combo seat for him instead of another convertible for the younger DS).
Anyway, we've been really happy with our Evenflo SecureKid 300's. They're really easy to install, reasonable (I got them at Kohls.com with Kohl's cash and/or 30% off coupons to make them under $100), and DS seems to like them.
It harnesses him now, but when he gets bigger he'll be able to use it as a backless booster. He's 39" tall and about 38 lbs. I don't recall the tallest height for the SK.
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DS - Adopted by loving parents 1995
DS1 7/2009 ('Stachio)
DS2 9/2011 (Peanut)