When I had more money to spend and lived alone, I didn't mind "winging it" and taking trips with only destinations in mind. I didn't check on restaurants or gas prices (of course, this was before this information was available in near real-time on the web).
This month I am going to experiment with a thorough budget including all probably costs (i.e. gas, lodging, food, tolls, entrance fees, souvenirs and incidentals) to see how close I can come to my actual costs on my family trip. I will post my findings here.
How robust is your pre-planning? Do you work out details on schedules, destinations, dining, lodging, etc. before you set out, or do you leave some wiggle room for spontaneity in some of those factors? Or do you leave most categories open and just hit the road?