Thanks for all your help and suggestions guys!!
-Have considered the 50hr check and planning to keep on the right side of it, with a few stops that can be left out saving a couple of hours if needed - but that said if it turns out to be a 100hr, we do have a few that can be added in too
-Grand Canyon was on the list very very early on - and we've also got some proper random places in a case of "that town name sounds familiar"
-Did consider Europe earlier in the yr, along with Africa, South Africa and Thialand - i think each one would have been fantastic - but it turns out america is the one which works for us.....this time round!!
Planning to keep a blog along the way - with plenty of pictures for added jealousy, so i'll post a link on here if it happens