Family fun is a dish best served impromptu. Oftentimes the planners of the family fun (i.e., all of us parents) focus too much on the destination instead of the journey. Headed to a theme park? Remember that the anticipation of something can be better than the thing itself. In other words, your kids are likely to be in a great mood on the way to their vacation spot, so make the most of it. Sing songs, play games and enjoy each others' company. No one will care if you don't make it in record time if you've had fun along the way.
Similarly, don't get upset if things don't go exactly as planned. Lines too long at the state fair? Take in a movie until the crowd thins out. Just keep in mind that "plan B" can be a blast if you keep an open mind.