Second the point that summer jobs, college orientations, and different start dates may make it difficult to plan far in advance. For my older kids, that was also complicated by obligations the younger ones had for their schools.
All these suggestions sound great! And also, as some have said, this is likley not the last vacation you will all have together. We had a trip to France with young adult kids which they really enjoyed (Normandy, the Brittany coast and Paris) as well as England when my youngest graduated college. However, they are not opposed to history and some art viewing and just being in Paris and London was exciting.
We also had great tripe to the Pacific Northwest (Seattle Area, Mt. Rainier and Olympic peninsula), which I was able to plan somewhat last minute. Also, Tetons, Yellowstone, and then Colorado.