Nothing against previous comments, but some of us don't have months to take off work and still want to get an overall impression of what's out there.
I'm currently planning a 4 week trip to the Southwest next year and there are going to be a lot of stops and some cursory sightseeing, after some long driving to get there. Four weeks is what I am able to take off of work, and I intend to come back in following years to focus more on what we liked on this first trip.
Best advice I got was to assess what your family can handle for speed / duration of a trip, and layout a general itinerary with a couple backup plans if you notice things not working out well. Practically speaking, I have a route setup on the Roadtrippers.com website with plenty of sightseeing options and then bailout options if the kids are starting to unravel. I think for us we can handle a few days of jumping from place to place and then will need a day where we don't do much other than hit the pool.