Leah wrote in with a hack I’ve seen used in restaurants all over. It never worked for us — our car seat never fit securely enough. Your mileage may vary, but it’s worth a try as long as it’s safe (check for a strap to buckle the seat onto the stand):
We have a two month-old, and just learned something that I want to share with the world: if you turn the wooden restaurant highchairs upside down, they accommodate an infant carseat! Safely! they are meant to!! Now, it could be that everyone knew this except us, but I told a friend and she didn’t know, either.
Safety note: Lots of Parenthackers commented that this is not safe, and there was even one instance of a waitress knocking into the chair and bumping the seat off its stand. Please be extra, extra careful should you decide to try this.