I'm biased but for domestic travel the western US, Alaska and Hawaii are truly unbeatable and by a wide margin imo. The only caveat to that is some of the historical stuff back east - DC, Civil War stuff and etc. So if I were you I'd plan another loop out here. California can get crowded but it is a beautiful and diverse state. Alaska or Hawaii would obviously involve a flying but they're both awesome. I travel back east and to the southeast a lot for work and I'm honestly glad that some of you guys love it, but to say it isn't my cup of tea is putting it mildly.
I would concur that the kids ought to be older before you tackle Europe. There's a ton to see there but the expense isn't worth it at their ages.
