15 years ago my wife and I decided to go for a 2 week drive thru the States with no particular destination.
When I got to the Detroit border and the customs dood asked my destination, I told him just that, I won't know where I'm going until I get there and didn't even know where we were gonna sleep that night, and that was a big mistake. I was more or less trying to be funny with the guy but he didn't see it that way.
He didn't pull me aside, but held us up, along with everyone behind us, for quite awhile with a million different questions.
I wouldn't tell any lies to those guys but would definetely give him the name of a specific destination.
Humour stops BEFORE the border