Ok...I asked my dad about this because he's spent a lot of time in Southern Utah...and this is what he came up with as must see areas:
Canyonlands ... in the northern part of the park, go see Muscleman Arch, Aztec Butte (its a bit of a walk to the top but there are Indian ruins up there) and go see Upheaval Dome. In the southern part of the park, Elephant canyon is a must see, and try to see Chesler Park. He said that all these places will be marked on an area specific map of the Park.
As far as Monument Valley, he said the best place to go is a place on the way (although I can't tell you what way he was talking about) ...it's called Valley of the Gods. He said it's not far from Monument Valley, so if you have spare time, it might be a good place to go.
My dad is a big walker, so some of these places may be a bit of a hike, but since he knows the area so well, I'd bet that these are good places to see. Hope this helped a little!
One more thing...Mesa Verde is awesome. Very good choice there
