Interstate 70 west of Denver, through the mountains is lovely scenery and Eisenhower tunnel is fun too.
I have not driven I 70 west of Moab, as I wanted to see the interior of Utah. I ended up taking US 191 to Utah 95 to Utah 24 - scenery is FANTASTIC on 95 and 24, and the roads were all but deserted. 95 isn't much help to get you to St George though.
You could take I70 to Utah 24 to Utah 12, which will route you through the Dixie Forest and Grand Staircase National Monument, should be beautiful. Pickup US 89 outside Bryce and head south to Zion NP, Utah 9 will take you to St George.
This is a route I've looked at for a future trip :)