If you go out to eat at a sit down restaurant, for the love of everything sacred, don't sit and eat and drink with your friends and let your kids sit in the next booth and do whatever they want. It's really annoying.
And while we're on the subject - if you go to this same type of restaurant and plan to sit and hang out for awhile - the polite thing to do is tip your server as though you are eating. You're taking up space that someone else would eat and tip from.
My list of pet peeves has grown since working at a restaurant. :-)