Agree on Valley of The Gods but do not agree on just driving by Monument Valley on the highway - and the fee is very minimal (was $5 / car for the day and I went in early for a morning photo shoot and back in late afternoon for late photo shoot all for the same $5).
Agree the NR is pretty and far less crowds / commercialization (think there might be reasons for that?) but you see but a fraction of the Grand Canyon. Looking right into the sun. You do not see the Grand Canyon in it's full and true beauty. And is that not why you are going there?
Blanding has a great one block museum.
Hovenweep has become really great over the last few years. The ancient part is, of course, the same but first time we were there the visitor center was a trailer; now a beautiful visitor center and great trail to walk around.
Colorado Nat Mon great suggestion. North (Island in the Sky) section is where you should go and forget the Needles section unless into fairly serious hiking and off road / shelf driving.
And I agree that hiking the Narrows is the best part of Zion. Except for the drive in from the tunnel on Hwy 9.
We love Cap Reef and there are some nice hikes; we will be there for a week this Fall again.