K.
In regards to the old ghost town idea... South Park City in Fairplay is an interesting place. It's a recreated old west town with lots of history presented. It's a very do-able drive.
Kind of a long drive, (maybe too long for a 4 day vacation but closer than Moab) but...
I love the narrow gauge train ride from Durango to Silverton. It doesn't go all the way in the off season, but it does run, and in fact the shorter trip can be nice for the kids. Both of our kids loved it.
Mesa Verde... always fascinating.
Glenwood Springs is a favorite.
If you're willing to drive as far as Moab, you could go a different direction and head to Mt. Rushmore. If you go there, be sure to try the Needles highway (great tunnels!) and the Custer State Park wildlife loop... amazing wildlife right by your car! Lodging there is quite inexpensive in the off-season. If you're willing to go a bit off the main path you can see the Register Cliffs where pioneers traveling the Oregon Trail carved messages in the cliffs for those coming after them. You'll be able to touch the ruts from the wagon wheels!
Have fun and let us know what you decide!