@howells Arcosanti before entering Sedona.
White Pocket near Page. (You can find local tour or rent a 4x4 jeep)
Bryce Canyon is totally worth of a detour.
Valley of Fire before entering Vegas.
In April I'm planning this trip over about 6-7 days, mainly for the hiking: if you have any super specific recommendations for places to stay etc. I'd love to know. (I don't plan to stay long in LV, just to fly back to the UK.)