Sometimes we use expressions whose meanings are obvious, but can be somewhat humorous if you stop and think about what is being said.
For example... When you go to a drive-through fast food restaurant and finish giving your order, they'll often say, "That'll be $xxx at the window." There have been numerous times that I've wanted to say, "How much would it be if I paid at the back door?" Think about it.
Have you ever considered throwing something totally random into the middle of your drive-through order? "I'd like a double cheeseburger, fries, a large root beer, a light bulb, and a small strawberry milkshake." Half of the time I don't think they're really listening. And this would be a good way to find out.
I've also wanted to have some fun at sit-down restaurants too. When you go up to the host/hostess and state your name and how many in your group, it would be fun to give them a... well... shall we say, a "different" name? For example.... Birthday, Surprise, Housewarming, Christmas, or Bachelor. So when they call you, they say (probably without realizing it): "Birthday party of 5." or "Surprise party of 3." I've wanted to give the name "Donner" (with 8 ("ate") people in the party, of course) a few times but think that might be going a little too far.