First, for one of your already planned trips, be sure to bring your bike on the Yellowstone/Glacier run. There is tons of outstanding riding around and between those two destinations.
As for your core question, here’s a few suggestions for your remaining week:
1. South Dakota Black Hills. You want hidden gems? This is the place. Hundreds of miles of excellent singletrack from all-day cross country rides to some fun gnarly stuff as well. There’s plenty of non-biking things to do as well.
2. Durango/Telluride/Cortez. Lots of great rides both famous and hidden. Absolutely killer scenery.
3. Flagstaff/Sedona/Prescott. Sedona can be hot in the summer, so it’s best to get out on the trails early–but there’s the best scenery in all of mountain bike land and the trails are unique, varied and always entertaining. If it’s hot, Mt Elden above Flagstaff has enough altitude to get you above the heat and it also has many miles of grade-A singletrack. Prescott has miles upon miles of rarely ridden, but excellent trails. Prescott can also get hot. Best to do this run early summer or late summer. Avoid mid-June through mid-August.
4. If you have the time to range further, consider Tahoe and/or Downieville. Unique, gorgeous, and loaded with awesome trails.
Of course, there are plenty of great rides between Ft. Collins and each of these destinations, so you can also hit some excellent singletrack on your way to and from.