It totally depends for us. Many of the times we've went out there's been some nice older couple nearby missing their out-of-town grandkids so they make faces and play peek-a-boo with him and then combined with an enending supply of Cherrios, puffs or yogurt melts seems to keep him content in the highchair. We also will use the paci clip to attach a teeth or something to him so he can't keep throwing it on the ground and shouting, "Uhoh!".
We've been taking him to restaurants pretty regularly throughout his life, though, so I think he'd just pretty used to it.
Another tip is that he tends to behave better in his stroller if there's a restaurant you can take that in.