Hey James,
so much to see and do. I was out there twice as you know and only scratched the surface. I envy this gentleman and his ongoing journey:
http://theoasisofmysoul.com/A few points to think about:
In the Massey books, Engineer Pass is rated a 4 and Ophir Pass a 3. But you're not going to get away from steep drop-offs anywhere in CO except for the 1 rated roads, which are basically tame gravel roads. In the area you are talking about (Silverton - Ouray) I took Hurricane Pass out of Silverton over California Peak and ended up in what is considered the best ghost town - Animas. Then Cinnamon pass back out to pavement.
I didn't find the drop-offs too intimidating, it was some of the tight switchbacks that had me white-knuckled because I was alone and didn't have a spotter. As for snow, you should be fine unless some freak early storm comes in. The prime offroad season for the CO mountains is August-September. Once you get into October it gets a little dicey. One final note, have lots of water handy when driving because at 12,000+, you do start to feel the altitude. You're in better shape than me but you may start to get dehydrated and a bit light-headed.
White Rim trail is definitely a 2-day excursion if you are going to be stopping and taking pictures like Roadie did. Rushing through detracts from it big-time. Also, there are some side trails along the way that lead down to the rivers in various spots. Oh, and you MUST reserve a camping spot ahead of time on a particular night. The best spots (halfway point) are usually booked very early.
If you can't work the 2 days into your schedule, then the Shafer Trail is awesome for a one day trip. You actually have to start with the Shafer and its crazy drop down into the canyon to get to the White Rim Trail turnoff. But if you continue on the Shafer, you will be treated to some excellent scenery as well. Ultimately it leads out to Potash and back to Moab.
The Grand Canyon. I have been fortunate enough to visit every major spot over the years - South Rim, North RIm, Tuweep. For pure adventure it is definitely Tuweep. Its isolated but not extremely difficult to get to, other than the distance. I did the trip comfortably in one day (hotel to hotel back on pavement) with lots of stopping for pictures. The only thing that would cause major issues there is rain ... the roads become "impassible". I met a couple of people at the rim and they said they were staying in the nearby campground so that they could take in the sunset which is spectacular I suspect. The campground is walking distance form the rim. I unfortunately did not plan for that.
Second choice is North Rim but definitely more tourist oriented - pavement all the way, lodges, parking lots, guard rails on the walking trail, etc. Different experience and view, more open and vast (like the South Rim).
Even though Monument Valley is the one in all the movies (My Darling Clementine, Eiger Sanction, etc.), it is within Native land and has been set up as a paid tourist area. I didn't go into the area, just beside it, so I don't know the exact situation but I believe that after you pay an entrance fee, you can drive a circular route yourself, or take an organized tour that goes into more remote areas.
Valley of the Gods is in the Massey books so I took that route. Its a very easy circular road but with plenty of scenery and interesting vistas. Reading an entry on Expedition Portal I saw that there are some very cool primitive camp sites along the route and staying overnight close to some of the rock formations is an experience unto itself with the stars, etc. Again, my plans could not accommodate that.
OK, if I think of anything else I will post again.
Old U-HAUL trailer lifted...
