Have you ever gone to a restaurant and been surprised to find out that they don't have highchairs? I have, too many times.
I started carrying my Phil and Ted's Me Too Chair ($50) with me everywhere. All I have to do is slip it onto a sturdy table and tighten the screws so it's securely attached. The lightweight, small chair fits right in the basket of my stroller so if I'm running out for lunch with friends, I can always guarantee that my lil one has a place to sit (other than my lap).