Lee,
You are correct about being early for fall colors. For that information, I have much of what you would need
here. That won't be of any use for you on this trip, however.
As for other places you can check out in that amount of time, you're still in luck. Easily the best closest location from Denver is
Mt. Evans, which is about a 1 hour, 45-minute drive to the summit. The upper section of the road will close two days after you leave. This location is great at sunrise and sunset, and is one of the finest places in the country for mountain goats as well.
The next popular answer would undoubtedly be Rocky Mountain National Park, though it is my least favorite area in the state given all the people who flock there and think that it's the best thing in the state. Still, like anywhere, it's quiet during the photographer's working times. Trail Ridge Road would probably be the best choice, though there are the ever-popular Sprague, Bear and Dream lakes. The trails will crowd up shortly after sunrise.
The last hot spot I'll give is
the Great Sand Dunes, which is a 3.5-hour drive. All photographers love shooting sand dunes, and there aren't any better spots on the continent for that. You enter a totally different world once you start walking out on them.
These three locations would be my top three suggestions that gives you plenty of diversity to choose from and will give you some solid pictures to bring home. Given the landscape possibilities at those locations, I'd probably suggest forgetting about ghost towns during your short stay, but if that's still a top priority,
this site will be your best resource for you to pick from.
Have a great trip out and enjoy your stay!